In the very last scene of the movie, and just before Blondie shoots the rope that is holding Tuco on the grave's cross. You can see a car moving in the background, screen right of Tuco's head.
When the camera crew were rigging the bridge to be blown up, the supervisor in charge of the detonation who didn't speak any foreign language thought he had been given the instruction to blow up the bridge and proceded to do it. Trouble was, the camera crew hadn't finished setting everything up and it was not filmed at all. Sergio Leone was (according to Clint Eastwood) the angriest anyone had ever seen him. As a result, the army agreed to rebuild the bridge for free and it hired a detonator who did speak the right language. This time they got the shot.