This one is easy to remember still because it caused me to stop playing for 6 months.
I was playing in a singles rec 4.0 league, and was 5-0 without dropping a set so far. My next opponent was at the bottom of my division, but that didn't matter on this night. I expected to beat him convincingly...
I didn't. I lost 6-2 6-1 and by the end of the match I was so furious I just started hitting as hard as possible to commit errors and just get off the court. I could not do anything right, at all, and had given up mentally. I usually never lose my cool, or typically care about winning as long as I play within my own skill set.
While I was driving home that night I realized how silly it was to get so upset over something that should be fun. I hung up the rackets for 6 months and decided to not to play until I was craving the "fun of the game" as I'd call it. Now I have fun every time I go out on the court and have improved to 4.5 by just being loose and keeping a cool focus.
Learned an important lesson that night.