Hi,

The fact that the magicmouse is tactile even for the mousewheel, it send wrong delta to the mousewheel JS event when you stop wheeling (because your finger is still on the wheel tactile zone, but you don't actually wheeling, you just raise your finger).
It send a NaN to be precise.

You have to test the delta with a isNaN function to prevent corrupted datas.

Regards,

Jeremy