"It was a terrible mistake" is a fallacy around harassment and abuse that harassment and abuse are perpetrated due to the abuser not understanding social norms, or not understanding their victim's state of mind or desires or similar.
This isn't a logical fallacy (it's possible for abuse and harassment to happen this way) but the common case is that an abuser or harasser is deliberately demonstrating, exercising and exploiting power over people. (This also explains how popular or socially skilled people may be abusive "despite" having other social strategies available to them that demonstrably work.) It is a tempting fallacy for many geeks, who see themselves as having poor social skills and making a lot of social mistakes; whether this is true or false for an individual it becomes a habit for them to analyze all bad behaviour by anyone as a dreadful mistake.
This fallacy is used for silencing, in that if all abuse and harassment is read as a terrible mistake, or needs to meet very high burdens of proof of intent, feminist reactions that denounce it as deliberate, or as a choice, are taken to be over-the-top and demonising of someone who is fundamentally innocent (although see also Intention doesn't matter).
This fallacy may enable abusers, in allowing both them and their allies to distance themselves from their actions.