Or accidentally have any background processes running which happen to use data... I imagine most likely whats going to happen is you'll be on wifi with like 18

Or accidentally have any background processes running which happen to use data... I imagine most likely whats going to happen is you'll be on wifi with like 18 tabs open and one of them will have a stupid flash ad and wifi will go down for 5 seconds but in that time the ad will refresh and sync a new video and all your bandwidth will suddenly be gone. :P