All the minor errors and continuity problems in Star Wars are simply a sign of the times in which the film was made, at least originally. In 77, home video didn't exist yet and Lucas I am sure never dreamed fans would be scrutinizing every frame of Star Wars 35 years later.

The head "bonk" was likely filmed during the time when Fox execs were pressuring Lucas to finish the production fast or be shut down. 3 units filmed simultaneously during the final week(s) and I am sure something as minor as the head bonk was simply deemed too minor to re-film. As time went on and it was possible to fix, maybe it was thought of as a mistake that gave the movie character. I don't know.

People forget that even adjusted for inflation, this was a low-budget movie. Things changing color or disappearing from shot to shot were due to budgetary restrictions as much as anything else.

I saw this film in theaters in 77 and I never noticed the stormtrooper hitting his head, or the shrubs that appeared in the Lars household, or the hundreds of other minor fixes fans feel a compulsion to change now. No one cared. It was fun to watch and that's what mattered.

Now, the music cue was lazy, but until a few months ago, I didn't notice that either.