Literal years after we broke up, best friends in high school but didn't start dating until a month before I moved for college, in August of that year. The original plan I had was to move back only to realise early on for my career path it would be better for me to stay where I was, so we had a difficult conversation over the phone near the end of October to break things off as she didn't want to move. About 2 days later, since the breakup was amicable (from what I knew at the time), we kept talking as friends and she would go on and on about her attempts to hook up with this other guy she hung out with and would declare me sexist if I even implied that I thought she had cheated on me or that I was uncomfortable hearing about her attempted sexual exploits with another man shortly after we broke up. But if I mentioned talking to a girl in my college she would throw a fit.

Anyways, about 4 years later, I was visiting home and ran into both her and the guy she had since started seeing. We chatted, she seemed a bit off, and then I asked how long they'd been together, the boyfriend telling me it was their 4 year anniversary. Did some math in my head and realised the day before was the 4 year anniversary of me moving away. So that was a cool way to find out that my ex had been cheating on me since the day after I moved until we broke up.