Or have a mental ex (although an office fling is an extremely loose definition of an ex) that contacts your wife on Facebook and makes accusations of infidelity with herself, and others, in order to try and ruin someone else’s happiness.
I had behaved impeccably so it didn’t bother me, and was quite amusing to have it explained to her that this behaviour was probably why she was ditched. Also, this was 2 years later, and ironically she was due to get married to the guy she was cheating on when we were “together”. It was very tempting to contact him on the big day to explain, with plenty of evidence, why he was marrying a cheating and accusing mental case. I didn’t though, because I’m a normal person. Caveat emptor.
Bit of a shaggy dogs tale, but the moral is: Don’t be THAT ex.
Move on, move up, make yourself better, find someone a lot better.