I watched both sequences shot-by-shot on my computer, and the dent wasn't there on the top of Jangos helmet when he was holding it on the balcony on Utapau over-looking the arena. But when he stands up after the horned beast attempts to run him over, there it is.
The sequence on Kamino when Jango bumps his head entering Slave1, was done in tribute to episode four when the stormtrooper bangs his head on the bottom of the door entering the control room. They could've easily edited out the trooper hitting his head, but instead left it and merely added the radio static sound to mask the "clunk" of his helmet hitting the door. 8)