SB-129 / Karate Choppers - S1-E14
Continuity mistake: When Spongebob throws the Krabby Patties on Squidward's head there are only 6 of them, later there's at least 8.
SB-129 / Karate Choppers - S1-E14
Continuity mistake: When Squidward is in the white place and crashes through the floor, he doesn't have his clarinet before he crashes through the floor. But when he is in the time machine, he has his clarinet.
SB-129 / Karate Choppers - S1-E14
Continuity mistake: SB-129: When Squidward opens the door to the time machine, the exterior side of the door has a handle at the left side and opens at the left, but the interior side of that door also has the handle at the left side, though it should be at the right side. Then when Squidward activates the machine, the exterior handle is now at the right side and it opens at the right.
SB-129 / Karate Choppers - S1-E14
Continuity mistake: In "SB-129", every time Squidward goes to answer the door to find Spongebob and Patrick there, the picture on the wall next to the door changes noticeably.
SB-129 / Karate Choppers - S1-E14
Continuity mistake: In "Karate Choppers" when SpongeBob sticks his arm through Sandy's phone, Sandy sticks SpongeBob's hand through the other end, then Sandy puts the phone down and SpongeBob's hand immediately disappears.
Answer: I believe this is the Season 6 episode "Spongicus".