Probably my first conscious experience with something that I now know to be "meta" was watching The Muppet Movie (1979) as a kid. I didn't understand the magnitude of all of the celebrity cameos (Who is Steve Martin? Who is Bob Hope?), and I didn't quite understand how Doc Hopper was going to supply a whole frog leg restaurant with the legs of just one frog, but I did perceive that the ending credits were gleefully breaking some kind of movie-making rule, and that finale was one of my favorite scenes in the entire film.
At the close of the film, Kermit, Miss Piggy, et al have finally made it to Hollywood and are preparing for their big show. Kermit sings the contemplative, Oscar-nominated "Rainbow Connection" as he walks though an under-construction movie set depicting scenes from the movie we have just watched. Gonzo, rising to the ceiling holding onto fake balloons which are recreations of the real balloons he had been carried on earlier in the film, bumps into another set piece, which causes a chain reaction of chaos that ends up wrecking the whole set and blowing a hole in the roof. As the stunned Muppets stare up through the hole, an actual rainbow flows into the destroyed studio through the hole, illuminating the cast as they sing the final bars of the finale. The camera, meanwhile, has been slowly zooming out to reveal that the small group of principle Muppets has grown into a large crowd containing practically every Muppet ever created, many of which had not even appeared in the movie.
This seems like the end. The words "The End" even fade onto the screen. A frequent movie watcher would expect the screen then to fade to black as the credits begin to scroll up the screen. What actually happens is that the character Jack, who has been trying to catch up with Kermit's group throughout the movie but always just misses them, bursts through the screen with obvious relief at finally making it. The camera then cuts to a theater viewing room, the seats of which are full of shocked Muppets (Fozzy's face is the best!) who, like us, have just had the movie they were watching interrupted by Jack's breach through the screen.
At this point, the credits do begin, superimposed over shots of the Muppet cast milling around the theater chatting casually. As a child viewing this for the first time, I was enthralled by the sequence. They are real! See! Here they are out of character just being themselves! I didn't know at the time that I was encountering a carefully constructed meta statement. I just knew I was fascinated by the Muppets just hanging out being themselves.
This style of interrupted ending had appeared just a few years previously in Blazing Saddles (1974) and Monty Python and the Holy Grail (1975), both extremely popular films which feature a sort of filmus interruptus as the movies are cut off - by the main characters getting arrested and by the theater and sets being rushed by a mob respectively - before they have a chance to finish. However, those movies leave me feeling unsettled and unsure. The Muppet Movie ending is jarring, yes, but then immediately reassuring.
Check out this video of The Muppet Movie finale (sorry for the poor quality). It picks up right as the rainbow comes down through the ruined roof and continues through the ending credits. The shocking moment in which Jack breaks through the screen happens around 48 or 49 seconds in.
If the embedded video above does not work for some reason, here's the direct link. Watch the finale here.