No Weenies Allowed / Squilliam Returns - S3-E6
Continuity mistake: In "No Weeneis Allowed", Patrick punches SpongeBob's left eye, but the black eye is on SpongeBob's right eye.
The Snowball Effect / One Krabs Trash - S3-E3
Continuity mistake: In The Snowball Effect, Squidward builds his fort between his and Spongebob's house later in the ending scene his fort has moved about 30 yards away.
The Great Snail Race / Mid-Life Crustacean - S3-E15
Continuity mistake: In The Great Snail Race, Gary wipes out into the tires then fire fighters come and put him out (still near the tires) but in the wide shot Gary is back on the track.
SpongeBob's House Party (aka Party Pooper Pants) - S3-E10
Continuity mistake: When Gary and Spongebob are waiting at the door, there is a sign behind him that says "pin the tail on the horace" (yes it says horace), and there is one tail hanging up, but when everybody comes in there are suddenly two.
SpongeBob's House Party (aka Party Pooper Pants) - S3-E10
Other mistake: When Patchy pops Minnie Mermaid's pool, water comes out from a different place.
No Weenies Allowed / Squilliam Returns - S3-E6
Continuity mistake: In "Squilliam Returns", the world's smallest violin changes position between shots.
As Seen on TV / Can You Spare a Dime? - S3-E5
Continuity mistake: In "As Seen On T.V", the hat on Mr. Krabs's desk has 2 lines on the anchor instead of 1.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: In Rock-a-Bye Bivalve, Spongebob's tie turns black when he is doing the chores on Patrick's second day of work, and then turns back to normal when he walks into the living room.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Continuity mistake: In MMIV, there is no phone next to the door when SpongeBob shrinks the pickles. Later, Squidward calls on a phone.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Continuity mistake: At the start, when Mermaid Man and Barnacle Boy run out of the Krusty Krab there are cars everywhere. When SpongeBob runs to try and get Mermaid Man's autograph inside the Krusty Krab there are no cars visible.
The Algae's Always Greener / Sponge Guard On Duty - S3-E7
Continuity mistake: When Plankton says, "Look at his daughter. She's as big as a whale!", the soda can on the table suddenly moves to the left.
Welcome to the Chum Bucket / Frankendoodle - S3-E2
Continuity mistake: In 'Welcome to the Chum Bucket', the sky is constantly changing in an abnormal way. First, it is daylight when Plankton takes Spongebob into the Chum Bucket. When Spongebob starts singing, the sun is either coming up or going down. When Mr. Krabs starts singing, look through the hole in the wall and the sky looks to be about afternoon. When Spongebob and Mr. Krabs finish their song, it's nighttime.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: The second time the chocolate guy chases Spongebob and Patrick, they run away and leave all their bags. But, in the next shot, they have their bags with them.
The Algae's Always Greener / Sponge Guard On Duty - S3-E7
Continuity mistake: The Algae's Always Greener: At the start, the old man is carrying a tray with just a Krabby Patty and fries, but after the old man gets the ketchup bottle Plankton zooms in on the tray, and in this closeup there's a fork and a drink with a straw beside the Krabby Patty, then they vanish.
The Algae's Always Greener / Sponge Guard On Duty - S3-E7
Continuity mistake: The Algae's Always Greener: After Plankton uses his machine and wakes up in the Krusty Krab office, he looks out through the porthole on the office door leading to the eating area, but the porthole on that office door vanishes when Plankton returns to the office with the two Krabby Patties. (00:04:25)
The Snowball Effect / One Krabs Trash - S3-E3
Other mistake: In 'The Snowball Effect' when Squidward attempts to make Patrick and Spongebob resume their snowball fight he throws a snowball at them which they avoid, and it puts out Squidward's fire inside. Squidward had his back towards his house and threw the snowball in the opposite direction.
Welcome to the Chum Bucket / Frankendoodle - S3-E2
Continuity mistake: The pencil's length in 'Frankendoodle' varies in between above and beneath the water surface.
Mermaid Man and Barnacle Boy IV / Doing Time - S3-E1
Other mistake: When the citizens pop out of Sponge Bob's holes near the end of Mermaid Man and Barnacle Boy 4, if you look closely, Sandy's head is gone except her eyes and nose.
Chocolate With Nuts / Mermaid Man And Barnacle Boy V - S3-E11
Continuity mistake: When Barnacle Boy and E.V.I.L. are hiding behind the rock you can see Barnacle Boy's fingers. In the next shot he is wearing gloves.
Krab Borg / Rock-a-Bye Bivalve - S3-E8
Continuity mistake: In "Rock-a-Bye Bivalve" when Spongebob and Patrick hear the bivalve chirping, the coral that vibrates is bright green and is standing up straight. However, when they find the bivalve and take it out from behind the coral, the coral is dark green and is bent over.
Answer: I believe this is the Season 6 episode "Spongicus".