Bugs:WWF No Mercy
This page details bugs of WWF No Mercy.
Save Data Glitch
Early copies of the game had an infamous recurring glitch that would erase saved data. Championship Mode and/or created wrestlers seemed to be the main catalyst(s), but the cause may not have been conclusively found. In any case, a revision that fixed this glitch was later released.
The PAL version also has this fix but awkwardly removed visible blood as well, even though First Blood matches and animations for reacting to bloodshed remained.
Get on the top of the commentary table using a character with Taunt 196. Do the taunt and a kick at the same time until the character reaches out of the table; the character will start floating, allowing you to go on top of the crowd, go outside of the arena, or get to the top of the stage.
Don't do an Irish whip or get Irish whipped, though: the character won't be able to stop running and will eventually go out of bounds, crashing the game.
Unnamed Wrestler Glitch
In the Hardcore Championship path, sometimes the opponent will be a strange semi-naked unnamed wrestler that has no entrance animation (he will be sliding in his fighting stance), music, or TitanTron; has The Rock's picture; lacks any moves or taunts (he will instead use the basic chop, but can't damage anyone); and can be easily beaten.
He also appears as "Data-2500" in the first slot in the unused Game Boy Color championship path when cloning a CAW into a slot, so he could presumably be a placeholder wrestler or the early model to start with for the CAW mode.