I have several but I will list 2.
Eagles concert and they announced no opening act but my friend and I knew there was another group. Everyone is screaming as the lights go down and the band says “we are Blue Steel” and it went silent. We laughed real hard .
Elton John had 2 guys playing cellos as his opening act. Then Elton’s band comes out and plays with them. After a few more songs Elton comes out and starts his show. No break from opening act to Elton
During Bob Dylan’s “religious phase” he toured and played in Pittsburgh. I stood in line overnight and got great seats.
The opening act was a gospel act! People were laughing, but the gospel group did another song, then another. Eventually there were boos.
Then Dylan came out and performed with the gospel band as his backup. He only did songs from his Train Running religious album. It was awful. I was close to crying (a combination of drugs and disappointment).
Around the same time I saw Rod Stewart in concert. The opening act was a band nobody had heard of called Air Supply. Their first song earned them boos and it got worse with each song. Nobody was there to see Air Supply.
Was at a concert in the parking lot. Saw a guy come up with an open wallet. I thought he was buying a ticket but he arrested a guy I assume for pot or some other drugs.
A copy was chasing a guy in the upper deck of the arena and someone grabbed the cops baton and threw it and it landed right next to me. I was not going to pick it up but someone did and ran off.
We saw Neil Diamond.
Middle of his show, the sound goes out … entirely.
Neil keeps singing for a minute but nobody can hear him. The natives are getting restless.
So he drops his pants, shuffles around the stage, pants around his ankles, and keeps singing – inaudibly – until the sound comes back on.
That’s a showman.
The event that “Smoke on the Water” is about: Smoke on the Water - Wikipedia
I read an excerpt of an interview with a witness, who explained why security didn’t intervene sooner: “It was a Frank Zappa concert in the 70s. A guy running around shooting off a flare gun didn’t really attract much attention.”
Saw Rod Stewart at the old Pine Knob near Detroit in the late 1980s or early 1990s. Rod jumped up on one of the amps while singing, slipped, fell to the stage and knocked himself out. He lay there sprawled out unconscious with the spotlight on him. The guitar players were in front of him, realized he wasn’t singing, turned around and saw him and stopped in confusion. The spotlight went off. Everyone in the place was dumbfounded. The band clustered around him. Then he got to his feet, staggered to the mike, muttered “There will be a short intermission” and left the stage.
About 15 minutes later he came back onstage and finished the concert. Props for his dedication to his art.
I remember some Rod interview from maybe the 70s, how he said he pours Coca Cola all over the stage because it gets sticky and keeps him from slipping.
I guess he should have put it on the amps, too.
I could tell about the concerts I went to where a different opening act than was scheduled, or how the main act was two hours late, but this thread is about unusual events. 
I saw Peter Frampton at the Chicago Theater in 2011, on his “Frampton Comes Alive! 35” tour. Early in the concert, in between songs, as roadies were on stage, adjusting things, a very attractive young woman walked onto the stage from stage left, walked over to Frampton, embraced him and kissed him on the cheek. I didn’t even realize what was happening until she hugged him (at first, I thought she was a roadie), and Frampton was completely surprised by it, too.
She was quickly hustled off the stage by security (who had, obviously, reacted a few moments too late), and Frampton started laughing. “She was very pretty…doesn’t she realize how old I am?”
Johnny Winter performed at the Pittsburgh Blues Festival a year or two before he died. I was a volunteer at the event and was assigned to be his “helper”. I brought food and drink onto the tour bus for him and kept track of the time. He told me he needed a good thirty minutes to reach the stage, which was maybe 75 yards from where we were.
When it was time to head to his stage, he had me get his hat (which he put on), a guitar (which he slung over my back), and a folding chair (which I carried). I helped him off the bus the way you’d help your grandmother who is 97 and just had a hip replaced.
Once off the bus he was winded. He had me set up the chair and he sat down. He sent me to get water. Once recovered, we started walking toward the stage with me holding his elbow to keep him steady. Although our journey was all downhill, we took three more breaks before arriving at the stage.
He performed seated, but he really nailed it. I watched from stage left and he was amazing.
Another that came to mind: I’ve been to several concerts at the outdoor venue which the city of Chicago built at Northerly Island, where the Meigs Field airport used to be. It’s a very fun venue, with the city’s skyline behind the stage.
One of those concerts was a triple-bill: Kansas, Foreigner, and Styx. The weather had been a little unsettled all day, with some light showers, but no serious weather. But, when Styx (which was the final act) was about 3 songs into their set, a cold front hit downtown Chicago, with very strong winds. The stage started to sway, as did the big amps which were hanging from the upper part of the stage – several roadies scrambled to try to bring the amps under control. A minute or two later, the venue’s management shut everything down, and cancelled the rest of the show; Tommy Shaw apologized to the fans, as the band sprinted offstage.
Same thing happened back in the 70s, huge storm.
John Sebastian kept playing. He got a roadie to find him a big fluffy towel, which he held along with his guitar pick, trying to dry the strings every few strums. As the rain pelted us horizontally off the lake he yelled “Wow, I thought only in England would people brave a storm for some rock and roll, but [maniacal laugh]… but you guys are crazy!” [cue soggy but heartfelt applause]
Not super unusual, but it was spectacular.
One of my favorite bands is Rasputina (here is a sample)- a sort of steampunky/goth/with cellos band.
One year, they had a concert at a venue with a wall of windows on one side. High ceilings (there was a sort of half second floor/balcony), so they were big windows and it was really a great space.
So halfway through the concert, a wild thunderstorm hits. Tons of thunder and lightning, sheets and sheets of rain, all visible through these enormous windows as this gothy cello band is playing. It even started leaking and running down the wall behind them and they had to move some of their equipment. Not awesome for the people who had to worry about the venue/cleanup, I suppose, but it was cool from the audience.
One of my favorite things happened at a Steven Stills concert. He announced at the beginning “Look, I was going to have a surprise opening act, but my friend Neil Young couldn’t make it. So if you don’t mind, I’m just going to play til midnight.” (he played longer, almost four hours’ worth).
But the favorite thing was, when he switched to acoustic guitar for a “classic blues” set, you could hear all the people who were still chatting. He stopped mid-riff and said "Y’know, I get a little nervous up here, so I always pretend I’m just playing in my living room for my friends…
… my friends don’t interrupt me."
I saw Bonnie Raitt in Pittsburgh at the old Stanley Theater on Bonnie’s thirtieth birthday. We had front row seats. At one point Bonnie accepted an offered bottle, took a swig, then handed the bottle back.
When she was supposed to finish, the back doors were all being opened. Ms Raitt yelled into her mike to close the doors, it was her birthday and she’d play till she was done. She did an extra 30 minutes.
My story is weather-related, too.
In the 80s, I went to a Depeche Mode Blasphemous Rumours concert at the Rose Bowl – Thomas Dolby was also on the bill, and another band I’ve completely forgotten. The Rose Bowl is set right up against the foothills, and thunderstorms are not unusual. This storm came out of a completely clear sky just as they began to play Blasphemous Rumours. Lightning , thunder, and everything. It was cool. Just as the song was over the storm stopped, and there were no more weather issues the rest of the night.
ETA: the opening act was Wire.
One more weather related.
I saw Boz Scaggs in Johnstown years ago. I was standing by the corner of the stage talking to one of the stage crew before the show started. He showed me how he was following a big storm on his laptop. Although the weather looked great, he told me 45 minutes in we’d get a rain.
The show was great. The crew dude let Boz know when the storm was due, and he did Rainy Night In Georgia. It was wild, everyone recognized the intro to the song and then a light drizzle started, then crescendoed into real rain as the song, and concert, finished.
This is unusual, but possibly not unusual for Lew Lewis.
I’ve have posted this on here before. but what the hell.
Lew blew a mean harmonica, something which will always make me happy. But rarely, if ever, have I seen someone so pissed (meaning drunk) on stage. He was draped over the mike, surrounded by pints of beer stood on the stage by his feet. Between songs he would drink, and amuse himself by tossing one of his many harps up in the air and catching it - until he missed one which, with unerring accuracy, plunged into a pint glass like an Olympic diver, making barely a ripple on entry.
There then followed the marvelous spectacle of Lew being utterly unable to figure out what had just happened and where his harp had gone; searching the stage for it, with the crowd shouting In your pint! When he eventually he realised what the crowd was shouting, he then had to inspect each pint in turn to find the missing harp, before success and considerable cheering from the audience.
I have just noticed - discovered while posting this - that Lew died within this last month. His passing saddens me, the moreso because the loss of this quietly important figure seems to have been almost unreported. He was one of the good ones.
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Was there really a band called Blue Steel?!
Googling indicates that there are (or were) several, but I suspect that the humor was in reference to the pouty expression which Ben Stiller used as a fashion model in Zoolander.
Saw that happen at a Tom Petty concert. Middle of the show something goes wrong. After a few seconds of diagnosis, they realized an amp died. It took the roadies about 5 minutes to pull the bad amp and install a new one (on stage, behind the band) while Tom played/sang, IIRC, This Land Is Your Land, with an unamplified electric guitar that we could faintly hear. Of course the entire crowd sang along.
Unrelated to the rest of your post, but this reminds me of a prank I saw on youtube a while back. For those that don’t want to watch the video, it’s John Mayer in a bear costume wandering the parking lot before his show. The tailgating fans don’t know it’s him so he goes around essentially taking trash about himself and the fans defending him. Whether he did it for the LOLs or as an ego boost, I don’t know, but it’s entertaining.