Here's one:
We weren't dating necessarily, just having consensual sexual intercourse...behind the back of her boyfriend. She was allergic to latex, latex-free condoms were pretty expensive for a college kid, so she would steal them from him and bring them over. Eventually they broke up because he backhanded her in the car while he was driving. I thought this meant she was done with him and now we were together. She invited me down to a wedding, I went down, she was acting really sketchy. We get back from the wedding and were hanging out on the couch when her new side piece shows up at the door. That was the end of that.
A few years later, we're hanging out as friends, we have a few drinks, head back to my place, hit the sack...I forgot she was allergic to latex and knowing she was a curious cat, I wasn't about to take the risk. We part ways. 5 months later she's engaged and taking engagement/baby belly pictures. Dodged a bullet.