For me it has to be the time Shelfie's Pegasus-mounted wizard cast Tanglefoot on my one and only war elephant. Effectively, the spell would have taken the beast out of action, as I was in no position to attempt a dispel.
The spell was cast, but with grim inevitability I rolled a 1, making the wise beasts magic res save.
Shelfie then preceded to throw his rulebook on the floor and leap upon it multiple times, simultaneously utilising colourful language to further illustrate his frustration.
How does power corrupt our hearts so surely?