deepanddark20 said:

Is there something about the inherent nature of stormtroopers that makes a particular stormtrooper bumping into something an impossible event?

Not impossible, funny. The stormtroopers are trying to do something very serious and menacing and the headbonk completely undermines that. It doesn't really matter if it's realistic or not. If a bank robber in the real world cracked his skull on the doorway on the way out of the bank, it'd be funny. If you were a policeman reviewing the bank's security tapes, tell me you wouldn't be tempted to rewind and watch him bonk his head again. Yeah it's slapstick, but maybe I'm just lowbrow.

The dramatic reveal of a planet where the whole planet misses its cue by a good few seconds is funny too. The thing that makes the latter clearly an error is that the musical cue was obviously intended, as shown by the '77 crawl. Yeah, obviously you wouldn't laugh if you didn't notice, but assuming you notice, it's funny.