The patty is very old and poisonous since it contains lots of bacteria and mold. It has rust all over it with green oil surrounding it. When SpongeBob takes it out from under the grill, he describes it as cold and hard and knows that it is quite possible that it rolled under there years ago.
Role in episode
SpongeBob finds the patty under the grill and he throws it away but is stopped by Mr. Krabs. SpongeBob tells Mr. Krabs that they should throw the patty away but Mr. Krabs tells him that he cannot make another Krabby Patty until the rotten patty is sold. Weeks later, the patty is not sold to any customer because it's already spoiled and inedible. Squidward angrily tells Mr. Krabs that the patty has gone bad but Mr. Krabs ignores him. The patty then barks like a dog while being kept inside a cage. To prove SpongeBob and Squidward wrong, Mr. Krabs takes a bite of the patty and then turns green due to food poisoning, being sent to the hospital afterwards.