She was supposed to do that. I'm watching the movie right now and she does that as a sign of knowledge, like you tap your head when you know something.
It didn't LOOK like it was on purpose. She wasn't even paying attention. If she DID do it purposely, dont you think she would have "tapped it?" She just moved her arm and it clunked it.
She definitely did NOT do it on purpose. However it's not even that big of a slip up...or that fun. And it didn't go "CLUNK" it was more like "ping"...she barely tapped it. NEXT! lol