It’s not a secret as such but very much a small world sort of situation and one I felt absolutely terrible for afterwards for a while.
I’m an academic and basically one day was conversing with a second year student about adults who love Harry Potter. We were having a 1:1 discussion, thankfully not in earshot of anyone else. I told her I had a friend at secondary school who loved HP so much she filled her parents dining room cabinets with the Lego, figures, other merch etc and they all went to HP world in Orlando annually, she even learnt parcel tongue.
She asked whether I still spoke to her and I stupidly let slip that I stopped talking to her in my early 20s because she was having an affair with an older married man she worked with and he had a young daughter so I heavily disapproved. He left his wife for her and I just felt too bad for the daughter in particular so stopped talking to her.
When I said the last part, she asked me how old I was so I said 31 or whatever I was at the time. She then asked if my friend was called “Kate” and I was shocked, said yes- how did she know her? She said “she was my Dad’s first girlfriend when he left my Mum…” Turned out she was the daughter I’d felt bad for a decade prior. Could see the cogs going and she said “I didn’t know he had an affair”. I felt like such a twat.
I took her for coffee and cake and kept awkwardly apologising. I mean, I couldn’t have known but yeah. Basically either her mum didn’t know it was an affair either or they’d chosen not to tell her and I’d just blown the lid.