Watch the scene when Manco (Eastwood), Mortimer (Van Cleef) et. al. are counting the time for the Bank Guards to walk around the bank. As the guards walk down the side of the bank, you can see them walk down the steps at the back left hand corner (as seen in the scene after the robbery). When they walk back into shot next, they are walking down the right hand side of the bank to the front corner, yet the count is continuous - hence they must have jumped from one back corner to the other back corner in no time.

I only noticed this when trying to determine the layout of the bank for a game. It is not easy to see at first, but is definitely there!