Nature Pants / Opposite Day - S1-E9
Continuity mistake: Opposite Day: The front door knob on Squidward's house repeatedly switches between the left and right side of the door, so it opens the opposite way (it has nothing to do with it being opposite day).
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: Mermaid Man wears green tights, but after SpongeBob absorbs the water ball like "some kind of evil sponge," when Mermaid Man and Barnacle Boy start doing the dog paddle, Mermaid Man's tights have vanished in this shot.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When Barnacle Boy opens the box with the two rings inside, in the closeup the "B" ring is on the left and the "M" ring on the right, but in the wideshot Barnacle Boy removes the ring on the left (the "B") and places it on Mermaid Man's finger, then in the next shot we see it's the "M" ring, before both rings vanish.
Mermaid Man and Barnacle Boy / Pickles - S1-E6
Continuity mistake: Mermaid Man and Barnacle Boy: When SpongeBob and Patrick (dressed in their superhero costumes) exit the pineapple the front door opens at the right side (outward), but after getting on Squidward's nerves, when they retreat back inside the front door opens the opposite way at the left side (inward).
Dying For Pie / Imitation Krabs - S2-E6
Continuity mistake: Mr Krabs has no eyelashes when he tells Squidward not to tell Spongebob about the bomb he ate. (00:05:01)
FarmerBob/Gary and Spot - S12-E1
Continuity mistake: In "FarmerBob" when SpongeBob is taken to Old Man Jenkins' farm, after that, the next shot when he is stretched about 50m tall by a bag of potatoes, the tie is missing. (00:00:50 - 00:01:25)
A Flea in Her Dome / The Donut of Shame / The Krusty Plate - S5-E11
Factual error: In 'A Flea in Her Dome' Sandy opened up the door for water to fill her dome. The water even replaced the air on the inside which was above the door, which isn't possible because the air would have to go down through the door to escape the dome.
Grandma's Kisses / Squidville - S2-E9
Continuity mistake: When Spongebob and Patrick first arrive at the crowd, there's a window on the wall next to them. After they mistake that one octopus for Squidward, the window has vanished.
Driven to Tears/Rule of Dumb - S4-E17
Continuity mistake: In 'Rule of Dumb' Patrick's house returned to its former self in the outside shot just before Squidward's house was moved.
The Thing/Hocus Pocus - S4-E16
Continuity mistake: In 'The Thing' the house of Squidward's door first opened outwards, then later inwards. It also changes color.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: In 'Chocolate With Nuts' the bandaged up fish's house's front stairs had only a few steps when Spongebob and Patrick approached, but as the fish falls down there's suddenly a lot more.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: In 'Rock-a-Bye Bivalve' Spongebob's front door changes hinge sides. Also at the end suddenly there's a light switch next to the door which wasn't there earlier.
The Secret Box / Band Geeks - S2-E14
Continuity mistake: In Band Geeks on day four the lever besides the big gate changes positions between pointing upwards and downwards several times.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Revealing mistake: In the shot where Spongebob says, "That little squirrel is in trouble, " half of his belt is missing.
Someone's in the Kitchen with Sandy/The Inside Job - S7-E3
Other mistake: In the kitchen when Plankton asks, "In approximately how long do you cook a patty?" Spongebob's eyelashes are missing.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: Spongebob's belt is missing when his clothes are first revealed while dangling in midair.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: The Krusty Krab is filled with Halloween decorations during the episode, but when Spongebob jumps to hide in a barrel there are no decorations in sight.
Scaredy Pants / I Was a Teen-age Gary - S1-E13
Continuity mistake: When Spongebob agrees to hear Mr. Krabs' story, their seats come out of nowhere.
The Secret Box / Band Geeks - S2-E14
Revealing mistake: When Squidward asks who has played instruments before and Plankton responds, look at Plankton's triangle. The bottom disappears for a split-second.
Selling Out/Funny Pants - S4-E5
Continuity mistake: In "Selling Out", Mr. Krabs hears the footsteps of many customers approaching and the shot shows the entrance to the Krusty Krab from the inside, with no one in sight. When Mr. Krabs tells the guys that they're coming, it shows Squidward in a few shots mopping right in front of the doors, even though he wasn't seen there previously.
Answer: I believe this is the Season 6 episode "Spongicus".