Factual error: When Clint Eastwood first goes to the barber shop, the barber very deliberately sharpens his razor the wrong way, flipping the blade with its sharp edge against the strop - this would instantly dull the blade. No real barber would make this mistake, but it's a common movie error.
Continuity mistake: Early in the bar scene, the drifter's bottle goes from his left to his right on the bar.
Revealing mistake: Before Eastwood shoots the tree dummies on the cart, the reflection of the window shows a clear blue sky. When the camera moves in front of the window, a black curtains' been placed to avoid the crew from being reflected. The curtain drifts all the time showing a tiny bit of sky, but for a brief moment you can see someone closing the curtain.
Continuity mistake: In the scene where Eastwood is watching "the city of Lago volunteers" training,he is drinking a beer. The first time he lifts the glass to drink, it is about half empty. After he says something to the man with the knife, he lifts the glass for another drink and it is nearly full.