How did your SO cheat on you?

The last time I was cheated on was when I was 18. I called my gf at the time because I had to deliver some bad news. She told me to wait because she had bad news of her own first. She told me that she had slept with another guy earlier that day and wanted to leave me for him. I couldn't help but laugh. I was just coming out of my ex's house, having cheated on my gf that same night! I was calling to break up with her! She didn't find it as humorous as I did, but it worked out for the best. Obviously neither of us were happy in the relationship and should have broken up long before then.