By that I mean... an impenetrable obstacle not near the end of the game, not an end or sub-boss. Just a level boss, level, or a mere enemy that always means it's "Game Over" for you whenever you encounter it.

A common answer for this back in the day would've been Bald Bull in Mike Tyson's Punch-Out!! Even if you knew the secret to his Bull Charge, performing it with precise timing was another matter entirely.

For me, the answer's easy: Vega (aka Claw) from Street Fighter II: The World Warrior and Turbo for SNES. I suppose he's a pain in any of the SFII games, but those are where I kept encountering him.

I never owned a SFII game but they were frequent rentals. No matter how much I studied Nintendo Power, no matter how many kids on the playground I asked for tips, it didn't matter... I could never make it past Vega. Of course his most infamous technique is his Wall Climb → Flying Barcelona Attack / Izuna Drop.

But it wasn't until recently that I learned of another reason why he felt so slippery: he can't block. Go ahead and try! Corner him, throw some strikes. What does he do? He'll just back handspring out of it, during which he's invincible.

The day I finally beat him was a good day.