Revealing mistake: At the beginning of Rock Bottom, when the bus pulls up in front of Patrick and SpongeBob, right before Patrick steps up onto the bus, SpongeBob's right arm is missing and it suddenly appears as he starts moving.
Revealing mistake: Near the end of "Rock Bottom", when SpongeBob arrives back to Bikini Bottom, SpongeBob's pineapple is animated differently from the other buildings. It looks like it was cheaply tacked on in post production.
Continuity mistake: In "Rock Bottom" when Spongebob is on line and the egg cracks, you can see the cracked pieces on the ground and then when he does his (in pants) "Barnacles, tartar sauce" you can't see the egg pieces.
Continuity mistake: In the scene where Mr. Krabs is saying "this is no game" Spongebob is in his underwear, then it goes to Mr. Krabs and back to Spongebob and he's in his work clothes.
Audio problem: In the episode "Rock Bottom" when Spongebob says "Another funtastic day at glove world," near the beginning, if you watch Spongebob's mouth, it only says "Another funtastic day at." It never says "glove world."
Continuity mistake: Rock Bottom: When the cardboard version of Spongebob next to the candy machine is shown, it is frowning, but it turns into a smile when it falls over. (00:18:48)
Continuity mistake: Rock Bottom: After SpongeBob and Patrick enter the bus, when SpongeBob asks Patrick if he has any change and for the duration of this shot, both the gear shift on the floor beside the driver and the open door of the bus have vanished, then they reappear.
Continuity mistake: When SpongeBob sees the kandy machine, the front of the machine is facing the road, but when he runs over to it, the front is now turned away from the road.
Continuity mistake: When SpongeBob and Patrick see the map, a compass is seen next to it. As the two start touching the map, the compass has disappeared.
Other mistake: Rock Bottom: When SpongeBob sets up a cardboard version of himself and is shown sneaking to the bus, his holes are missing and he is perfectly square, instead of his usual sponge-like shape. (00:18:47)
Answer: I believe this is the Season 6 episode "Spongicus".