Early Simpsons is full of this. You had to freeze-frame your VCR to see a lot of stuff (I remember a bar at the marina called “Smoke on the Water” that was way too quick to see in real time as Homer sped by it).
The mother ship in Close Encounters has some silly stuff on it.
In the movie Local Hero, a maniac on a moped crops up from time to time with no explanation whatsoever.
They’ve even made fun of their own tendency to do this. There was one episode a few years ago where there was a map with all kinds of weird names and stuff. The map filled up the screen for about an entire half minute, and Homer said something like, “did you read them all yet?” to the viewers.
In the Wreck it Ralph thread I mentioned my delight in noticing a brief flash of graffiti reading “Aerith Lives.” When I looked on the Internet to see if I saw correctly I found that there was also some graffiti which read “All your base belong to us.”
Right, it’s put front and center a few times. But multiple times during the episode you see him getting further hammered. Which is what makes the whole thing extra funny.
I want to say that sounds like it would have happened in “Airplane”, but can’t be certain.
There was a Big Bang Theory episode with a scene at the comic book store. Prominently displayed on the counter was a QR code. For the heck of it, I paused the DVR and scanned the code. It took you to an online store where you could actually buy shirts and games displayed in the store (http://www.thecomiccenter.com/)
On Stargate SG-1, the authors of books seen on screen, and other printed media often includes the names of directors, writers and producers on the show. Also photos of the crew are used for instances where someone is looking through a pile of personnel files.
Peter DeLuise and Martin Wood make several cameos in episodes they’d directed.
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Fox Mulder pees on an"Independence Day" poster in the first X-files movie.
You mean something like Elvis appearing as a background extra in Home Alone?
In the opening scene of Don Juan DeMarco, Johnny Depp romantically seduces a woman sitting alone in a hotel restaurant in the first scene, then leaves her, kissing her hand, just before her husband arrives. Later on in the movie, there’s another scene in the same restaurant. In the background, you see the woman sitting at a table alone in the background, looking around longingly.
About a year ago I was watching an episode of Malcolm in the Middle for what was probably the 10th time and I just happened to notice a cockroach crawl across the wall in the background. No attention was called to it, it wasn’t an episode based around how dirty they or their house was and yet someone had to bring in a cockroach, put it on the wall, get it to crawl up through the frame and get the actors to do the scene on cue with the roach just for the handful of people that might notice it.
In my first post, I forgot to mention that all this happened on an airplane, but it’s good that someone figured it out despite my omission.
I know for sure the scene wasn’t from Airplane or its sequel, because I’ve seen them a million times.
On an episode of Babylon 5, in the background of one scene was an electronic display listing the day’s activities on the station. One of the items was “Rolling Stones Farewell Concert.”
I couldn’t think of any other setting that would use overhead compartments. 
Check out Patton Oswalt standing perfectly still throughout this entire scene in King of Queens
There an entire website devoted to the pineapple in every episode of Psych.
Also, isn’t there a Han Solo figurine hidden in many Firefly episodes?
If I remember correctly there was also a “LeRoy Jenkins” tag.
Also a “Turbo (the main villain) Lives” tag that is seen shortly after he’s mentioned the first time.