At the start of the movie, a group of men are beating another man and are about to set his wagon on fire. Clint Eastwood appears and throws a bucket of water over the guy with the lit match putting pay to their arson plans. Only problem is if you watch closely the water somehow completely misses the lit match and the guy has to quickly put it out himself.
This is not necessarily a Slip-Up. A guy who gets a bucket of cold water on top of him would probably have a hard time to keep a match lit. Even if he did manage this, he probably would have turned it out anyway while finding out who threw the water.