The Offspring, Cobo Arena, 1995.
I’ve never smoked pot at all, but I do remember the first show I saw pot openly smoked at: Stooges reunion tour 2005 at Bumbershoot in Seattle. Why do I remember it so well? Because they played this song when people started lighting up in front of me.
Woodstock, 1969, when I was 17.
During one of the rain delays I took shelter under a plastic sheet some neighboring folks had and partook when the joint came around. I didn’t get high, though, probably because I only took a couple of hits. The first time I actually got high was almost a year later as a freshman in college.
The only time I ever smoked weed at a concert (and the second and last time I did it in my lifetime) was at a Van Halen concert in 1982. Worst concert I ever saw, too, although this wasn’t why.
It was so terrible, I almost didn’t see Rush at the same venue a month later (and no, they didn’t play “A Passage To Bangkok”, either - or at any of the other 3 shows I saw by them in later years!) 
Dave Matthews Band around '01 or '02.
Rolling Stones 1981
Does jazz in an intimate venue count? If so, McCoy Tyner, 1988.
If not, then the Grateful Dead (I know, cliche), I think in 1989.
September 1970, San Diego Sports Arena, Jethro Tull opening for Led Zeppelin.
First concert together for Mrs. Cretin and Moi, also first concert where either of us smoked boo.
Amazingly, in a crowd of 10,000 people we bumped into a mutual friend, a guy who always seemed to have access to high quality hemp; he gave us a joint to add to our larder. We were already baked and should have held back, but went ahead and smoked the super-J, and the next thing I knew I was home in bed the next morning. I could remember absolutely nothing about the night before post-J, making Tull/Zep also the first and only concert at which I ever got so wasted I missed the show.
Ooooo. For the win.
Does a contact high count? My first memory of walking into a fog was when I was 12 and my friend Mike and I went to a screening of The Song Remains the Same. We walked in and I remember thinking “hmm, what’s this smoke and smell? is this The Pot?” I didn’t get the movie, but when Jimmy Page pulled out the sword and waved it into a rainbow, it was cool.
Not really a concert, but still,
Laserium.
I was at my first concert, Van Halen, back in 1980.
July, 1972, L.A. Forum, seeing Ten Years After and Grand Funk Railroad.
Gawd, I feel old …
Queen. Cleveland Coliseum. January 1977.
I thought you can’t do that any more, because of tighter fire regulations these days. The days of saluting a performer with a crowd full of lit lighters are now just a memory.
Jethro Tull, Palace Theater, Albany, NY, November 1971.
Ozzy Osbourne. Motley Crue was the opener. '82 or '83?
I’ve never smoked pot in a concert; however my first experience with it was at Billy Joel concert in 1979 or 1980 when Big Shot was a huge hit. Some dude in front of me had Hulk TShirt on, got stoned and drunk off his ass and missed most of the concert because he passed out. i thougth to myself; “that didn’t seem like a smart thing to do.” As a result I’ve never really smoked pot. Thew few times I have, I was also drinking a lot, and ended up getting sick. Negative Reinforcement I guess.
Ugh tell me about it. There is something wrong with a bunch of cell phones now replacing lighters. Even worse is the cell phones using the ‘lighter’ app! Very sad. Very sad indeed.
Frank Zappa, 1972. A joint with hash oil on it.
I’d had plenty of experience with pot, but this was the first time at a concert.
I’ve never smoked pot, but the closest I’ve ever come was inhaling the background smoke of the people sitting in front of me at a U2 concert in Kansas City in October, 1992.