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?
Patrick SmartPants/SquidBob TentaclePants - S4-E8
Plot hole: In 'Patrick SmartPants' Patrick lost the top of his head after falling off a cliff, but Spongebob put it back on. Suddenly Patrick's intelligence skyrocketed, but wanted to return to his former self in the end of the episode. To do this he re-enacted his fall, but it was unsuccessful. They eventually figured out that Spongebob had in fact not put on Patrick's head, but a 'brain collar'. How did Patrick posses his memories throughout the 'intelligent phase' despite not having his own head (and brain)?
Plot hole: In "Once Bitten", Mr. Krabs never unlocks the doors to the Krusty Krab, but at the end of the episode, Squidward enters with no problem.