Most Helpful Girl
It could be anything from lying to being physically abusive (or anything else you'd consider "unforgivable") with a partner that one would consider an unforgivable mistake. Everyone is different and some could forgive all mistakes or some could forgive none. It's all a matter of how you feel if you would rather take a leap of faith and give them a second chance, or to never trust them again. I wouldn't suggest harboring unforgivness for the person even if you don't give them another chance, because it ends up doing more harm to you than to them, as it causes feels of negativity and stress. Though it is possible to "forgive and forget", as they always say, the choice is yours.
Most Helpful Guy
I would never forgive a girl for cutting off my legs while I sleep. untrustworthy.