Bruce my Machop in Pearl. I was strolling around Solaceon, battling some trainers or wild Pokémon. After one of the battles, I had to go heal. Bruce had ended the battle with less than a quarter of its HP left, and only 96 exp. points left before levelling up. Exiting the Poké Center, I figured that since I already was in Solaceon, I could go explore the Solaceon ruins for some loot.
I sprayed myself with repel and went inside to do some graverobbing. On my way out, my pockets full of loot, in the last or second to last chamber before the exit, the repel's effect wore off. I then met an Unown-D.
"Perfect, I can take it for those last Exp. Points," I thought to myself and ordered Bruce to attack, despite the type disadvantage. Bruce was in perfect health, and while he wasn't strong enough to OHKO, he outsped it, was several levels higher, and his moves would easily finish the puny Unown in two hits. I mean, Unown. The decidedly worst of all fully evolved Pokémon. What was the worst that could happen?
Critical hit STAB 50+ BP Hidden Power Psychic, that's what. Bruce was defeated in a single blow.
I might not be the first Nuzlocker to lose the prime battler of my team to an Unown, but I can't imagine that there are many of us. It might have been short-sighted planning on my part, but some of it certainly boils down to bad luck. Not only was the Hidden Power the only type Unown could get STAB from (1/16 chance), but it was also at a high Base Power (upper end of the 40 different powers HP can have, though I'm not sure where the treshold is), AND it critted (1/16 chance). The probability of HP Psychic and Critical Hit alone is 1/256. Multiply by the probability of it having high enough BP to KO, and you'll probably wind up somewhere in the 1/500 - 1/800 range.
Needless to say, I was slightly pissed.