Game running in web player, crashes on device

Hello! I'm basically done with a game, and so I put it into the simulator, and... it crashed. Which I find strange, because it ran fine on the web player. Anyways, has anybody else ran into this problem, and have you figured out what makes the simulator work differently than the web player? I'd really like to get this sorted out. I'm not sure if this helps, because I don't even know what it means, but this is what the simulator says:

"?: -1: attempt to index a nil value (upvalue '?')
stack traceback:
?: in upvalue '?'
?: in function <?:3764>"

(Edit: I put a "fin" in a weird place and somehow that fixed the problem, but if anybody knows, I would still like to know why the simulator and web player differ.