The Nasty Patty is a food served at the Krab Krusty. It was originally served to kill the health inspector, but Spengbab noticed how easy and cheap it is to make so it is now served along with the regular Krap Patty. The Nasty patty is made with ingredients so vile, it kills anyone that eats it within a few seconds.
Plankton, being the idiot that he is, tries this steal the formula for it, despite the fact that it can be found online and that no one ever eats it. Despite this, he will never give up trying to steal this seriously nasty piece of crap.
The Nasty Patty has only sold 1 patty and that was Plankton trying to steal it.