When I say mistake or error, it could literally be something the rapper said was factually wrong, mispelled or mispronounced a word (and not for cadence purposes, they really said the word wrong by accident), issue with the production (sound quality, skipping, drums are off, etc...) or it could be background sounds that shouldn't have been left in, etc...
An example Kanye West: "I'm in Too Deep like Mekhi Phife"
An example Kanye West: "I'm in Too Deep like Mekhi Phife"