Im also a triathlete, consequently much most disappointing result must be my DNF at the Hever Castle half distance ironman (middle distance triathlon).
Having had a disappointing result at full distance the previous year and a new addition to the family, I swore off Ironman, then signed up for this half distance. Determined to get a good result at this very hilly bike course, I focussed my effort into the bike, TR, bestbikesplit, the works, I even took a job near the course (!) so I could practice on the bike course, and even though it’s about an hour drive I camped there the night before so I was as prepared as I possibly could be!
So I come out of the water dreaming of a 3:30 bike split, and what I though was a realistic 3:45 goal, pumping up the hills watching my power meter like a hawk, hammering the downhills just the same, suddenly at about 45mph descent approaching a red flag turn my rear tire gives up and the rear end is washing all over the place, I hold it and free wheel as best as I can to a natural stop, lose fifteen minutes repairing the bike due to lost tools, remount and start hammering again…now it’s over 32°C, I’ve got many hills to climb and I’ve got a full closed TT helmet on…I am cooking hot.
I got hotter and hotter until I just couldn’t take anymore, stopped, threw my helmet on the grass, raged at the clouds whilst bemused triathletes plodded past, 
raged at the rules that meant I had to put the helmet on again, and got on with it.
I think my bike split was ~4h15 against my PB of 2h58. I was dead. I looked at the T2 tent. I looked at my watch. I thought about the half marathon ahead of me. “Nah.”
Because I quit at T2 and I was dead on my feet, the only place I had to sit was the finishers area, so I also had to deal with all the congratulations and backslapping 