Plankton's size varies throughout the series.
When Plankton turns the thermometer in the Krusty Krab, he turns the knob back and forth. The temperature goes down, not up and down like it should be.
Squidward plays his clarinet straight out, like an oboe, instead of angled downwards.
At numerous points in the series, the phrase "Galley Grub" on the Krusty Krab menu is misspelled as "Gally Grub."
The size of SpongeBob's bed varies throughout the series.
The Graffiti that says "Nematoads are People too" should read "Nematodes".
The cooking surface that SpongeBob uses is actually a griddle, yet it's always referred to as a grill. A griddle is a flat cooking surface, but a grill is an uneven cooking surface often found outdoors.
The location of the Krusty Krab changes in various episodes. Most of the time, it is located to the left of SpongeBob's house. Other times, it's located to the right.
The geography in Spongebob's house and Bikini Bottom changes almost every episode.