The Rusty Patty is a patty that only appeared in the episode "Born Again Krabs."
The patty is very old and is poisonous since it contains lots of bacteria and mold.
Role in episode
The patty was not sold to any customer, but Mr. Krabs attempted to sell it. The patty even barks at SpongeBob. Mr. Krabs later took a single bite of the patty and had to go to the hospital.