I have a couple:
1. A regional meet during my junior year of XC. Was super muddy and I fell once. I was trying to keep up with my teammate down the final stretch but I just had NOTHING left and my teammate always has this humongous kick and there I was just jogging it in... this guy from this other team is coming down fast down the final shoot but I just don't care and let him pass me which just looked ridicuolous. then I proceeded to collapse and vomit. It was a time where I kinda was still in the stages of my eating disorder so I ate absolutely nothing before than 10am race... I still subbed 17 but it was a terrible experience.
2. dropping the baton in the 4x4. 3rd leg and it completely slipped off my hand. Enough said.
3. I was doing a half marathon with my dad. He's honestly in better shape than I am (he's only 44) and I had to walk around mile 11.5 because I hit the wall around mile 8 and just couldn't keep going.... I just let him go on and finish the race.