Continuity mistake: In "New Leaf", When Mr. Krabs throws Plankton's snow globe in to the room, there is no lock. In the next shot, there is.
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", Squidward is standing at the register, holding a newspaper. When it cuts to "Patricia", and back to Squidward, Squidward isn't holding the newspaper anymore.
Skill Crane/Good Neighbors - S4-E4
Continuity mistake: In "Good Neighbors", Squidward stomps on Spongebob and Patrick's fezzes in front of his door. Then, it cuts to a shot of Squidward's house, but the fezzes are gone.
Fear of a Krabby Patty/Shell of a Man - S4-E1
Continuity mistake: When Mr. Krabs drags SpongeBob back into the kitchen, Squidward's facial expression changes between shots.
Plot hole: After being spared, Medieval Sandy announced that she will help in the defeat of medieval Plankton. How did she know what they were planning?
Squidtastic Voyage/That's No Lady - S4-E15
Continuity mistake: In "That's No Lady", Patrick runs through Spongebob's wall, leaving a Patrick-shape in his house. Patrick pushes a bit of Spongebob's house, which disappears in the next shot.