Mitchell is in the movie Casablanca, as an extra in the bar. Mitchell can't actually be seen. But he does knock over a chair at one point and one can see that.
Toobworld found three scenes that match to Mitchell's description:
- Rick's flashback to his time in Paris with Ilsa.
- Scene when Yvonne comes back to Rick's Place with a German officer on her arm.
- Scene when Victor Lazlo leads the band and the French supporters in the bar in singing "La Marseillaise".
More detailed, checked by Anonymous:
No chairs fall in the flashback of Paris. There is a chair knocked over at Rick's table in the present time, but there are no extra's in the bar in this scene as he is alone with Sam.
When Yvonne is at the bar with the German officer, no chair falls on screen. We hear a lot of shuffeling and moving of chairs when the fight happens.
When Victor Lazlo asks the band to play, a sound can be heard that could be a chair falling over. When the people are moving around some chairs can be seen tilting, though none are actually seen falling.
Another option: In the scene of Ugarte's arrest, right after the shots are fired, some chairs are knocked over at the table of a few men in uniform. Perhaps there's someone at the table we don't see.