When Indy starts running down the huge hallway away from the boulder, you can see some of his moves are repeated; the boulder, which has just rolled to the bottom of the track by the doorway, has now run back up it again about halfway to roll down once more, and both times as it closes in on him he looks over his shoulder as he runs and turns back to run full pelt away from it. If you look you can see that the moves are exactly repeated and the boulder jumps positions. This is all due to a small error in editing, as the sequence was all shot in one take with multiple cameras. They use the same piece of film but from different views. You can also see later, just after he trips and the camera view changes, that the boulder jumps closer behind him - too close in fact. Another bit of careless editing; when it closed in was edited out, and so just jumped. Duh!