Weirdly the "past" happened whilst I knew them.
Group of new recruits, like the one woman more than the men. Gradually become closer to one of the men and dislike the woman as quite brash and insensitive.
I was knee-deep in a project so mainly stayed away from them though, and missed an annual event that all the newbies went to. If there was a weird atmosphere in the office the day after I don't remember it.
A few years and a lot of additions to her record for poor performance, attitude and behaviour, I'm sick to death of her. Talk about her performance with her manager as she's affecting the whole team.
A few days later, there's a serious and substantiated claim of racism against the woman.
It's only then I find out the full story of the event. She got super drunk on work time and racially abused a black staff member at the event. What's more, the management at the time forgave her this, and kept her on even though it was her probation period!
I fired her.
It didn't just change my perception of her, it was the nail in the coffin for my perception of the other managers.
(Funnily enough another guy in her cohort was the only person they EVER failed the probation for - and he was pleasant and mild-mannered, and curious about the job!)