I believe the episode in question (yes, one of mine) was entitled
"Take Two," and ended with actual footage of the movie.

The wonderful part about that episode of TRGBs -- and this is
something I kinda aimed at while writing it -- is that there are now
whole bunches of kids out there who are absolutely *convinced* that the
animated series came first, THEN the movie based on it.

The only thing better than messing with somebody's sense of reality
is messing with a whole LOTTA people's sense of reality....

jms