Hmm.
Difficult..
Well, I remember dropping out of a 3000m once. It was the UK women's league and the race was in Swansea. It was an incredibly hot day. I had also run the 800 or 1500, can't remember which although think it could've been the 800. Did not feel well. Maybe I was dehydrated from the journey, don't know but I remember stepping off the track on the back straight and I rarely DNF'd (that might've been the only time).
The national XC championships one year, the team bus was late and got there minutes before the start of the U17 women's race... we were having to put on our spikes on the bus, unable to walk the course or warm up. I didn't run very well. Think the no warm up was the biggest factor as it was so cold and the race started very fast.
One time I remember that although it wasn't a bad race, I was very disappointed - I thought I had won, and it turned out I was 2nd woman. The woman who won had very short hair and wore loose running shorts (unisex ones). I spoke to the race organiser and apparently it had happened before. I spent pretty much the entire race thinking I was winning.
Ran some slow 400m but don't regret those as they were for fun, but objectively they may have been the 'worst' time wise.