Maybe there were issues with the console. Maybe there were issues with the programming. Maybe the spriter was on something when he drew up the sprites. Either way, what are some things you've encountered while trying to hack, sprite rip, or extrapolate values or mechanics from a Castlevania game, from whichever console? What has made you pull your hair, bang your head on the desk, or even induced fits of crying? Naturally, I'll start.
The NES has many limitations, so it takes some craftiness to create special visual effects while working within those limitations. But discovering those nifty tricks is, well, not something you necessarily want to ever accomplish. Case in point: Axe Knights from Dracula's Curse. While I was focusing on the placement of the axe at different stages of the throwing animation, something eventually caught my eye. At first I didn't notice it because it looked so natural, but then I remembered what I was working with. When the Axe Knight throws his axe while standing up, everything's hunky-dory, but when he kneels to throw his axe low, things go crazy -- his back foot gets amputated! It's actually a nifty trick because it gives the overall sprite much more depth, but if you're trying to rip sprites and don't notice it (which the compiler of the sprite sheet I'm using didn't) or recreate a game as closely to the original as you can, that little quirk can make you just drop your head in despair. It's a perfectly legal amputation -- a whole sprite (8x8px square) gets shifted one pixel back when he kneels and then shifted back into place when he throws his axe, giving the effect that he kicks off with his kneeling leg to put power behind his throw. More so than the popular pixel bob (which the Sword Skeleton uses to make its walking animation more, well, animated), I think this kind of spriting prowess is the most irksome.
Although there is the 15x16px+8x8px triple-decker spriting used in Bucky O'Hare that ranks up there as very annoying handiwork, but this is a CV forum so I won't go there.