Raw Sewage is a burger made by Plankton, to finally make a Krabby Patty without always failing. It only appears in the episode "Patty Caper." In that episode, Plankton tells SpongeBob that he is on a list in the alphabet, and that he is on the letter R, and to steal the Krabby Patty Formula.
At the beginning, SpongeBob thought that Plankton stole the secret ingredient for the Krabby Patty, until he finds out that the burger is not a real Krabby Patty and just a failed duplicate attempt.
The Raw Sewage burger looks similar to a Krabby Patty except it is darker in color. It also has dark green sauce on it which resembles raw sewage. The ingredients look much more decayed and the patty contains a strange odor.
After SpongeBob and Patrick left the Chum Bucket, Patrick must have eaten the burger, as there was bad breath smell coming out of his mouth.
The Raw Sewage Patty looks very similar to the Nasty Patty created by SpongeBob and Mr. Krabs in the episode "Nasty Patty." However, this patty was made on accident rather than purposely because Plankton was experimenting to find out the Krabby Patty formula.
This is not Plankton's first attempt, but one of many.