I once used over 7GB on an O2 simplicity montly contract. No that's not a typo. 7GB. I was moving houses and had to tether my mobile while my ISP took their sweet time installing the connection in the new place.

I didn't get charged anything extra. Although I did get warning texts from them by the end of the month. This was a 1 time thing but I normally end up going over the 250MB fair use policy per month anyway.