Kinda linked to my 'hardest challenge' thread, when have you gotten really fustrated with a game? I'm talking controller hurling, "KHAAAAAAAAN!"-screaming fustration?
Doesn't have to be a boss, can just be a really annoying jump in Mario or something.
My current number one is Fire Emblem on GBA, just beat all the enemies on level 16x, and I'm training everyone up to high levels at the arena.
After spending about half an hour on this, I send my mage into a fight he should have no trouble with, only to have the enemy rip a one-in-a-hundred critical hit on him, which he survives, retaliates, and gets killed by [/i]another critical hit before I can pull him out.
This not only kills the only mage I've got, but also saves the battle as it starts, forcing me to repeat the entire level.
The GBA went on a little trip across the room for that one...