Revealing mistake: During the driving scene, the background is illuminated by non-existent headlights. It's running backward on the projection screen.
Revealing mistake: Near the end the psycho killer pays a black man to beat him up so that he can then blame the injuries on Dirty Harry. Watch the killer's hairdo as he gets punched and beaten: a couple of rear-view scenes show an obviously fake wig, intended to look like the killer's own bad-hair-day style. (01:16:05)
Revealing mistake: When Shaft and his buds are sneaking into a building to save Bumpy's daughter, a pedestrian momentarily comes into the shot and then quickly steps out when he sees the camera crew. (01:18:45)
Suggested correction: I believe the entry refers to the initial rescue attempt, at 1:18:45. I am not entirely convinced that this is a legitimate goof though: the man sees 3 guys peeking into an alley, and he seems to rather stop and backtrack to check out whatever they are looking up, the exact opposite of 'stepping out'. It's still a rather amusing detail, though.
Revealing mistake: When David is re-loading his gun whilst out duck shooting, we see three ducks flying to the right and then immediately to the left, revealing that it is the same film in reverse.
Revealing mistake: After Duke hears noise coming from the stairwell he locks the doors at the end of the hallway. After Duke shoots Parelli it cuts to the police firing at those locked doors, and when the doorknob blows off we can see that it's only a glued on doorknob, with no hole in the wood door, so it's impossible to have actually locked it.