Mistakes
In "Jellyfishing", in the beginning, when Squidward walks out of his house with his bike, Spongebob says "Hey, Squidward" but his mouth is saying something different. See more...
Trivia
In the episode "Karate" when Sandy is about to pour the hot sause over Spongebob's tongue, the face on the sause that laughs and taunts Spongebob is Tom Kenny, the voice of Spongebob. See more...
Movie Mistakes blog
Popular blog posts:
Top 15 biggest Harry Potter film mistakes
The 10 biggest mistakes in Iron Man 1 & 2
15 biggest mistakes in Titanic
Other great sites
SpongeBob SquarePants (1999) - 5 with pictures
starring Bill Fagerbakke, Clancy Brown, Rodger Bumpass, Tom Kenny (add more)
Screenshots of mistakes that have been online for less than a year are only visible to members, but the rest are available to everyone. Don't forget to check out the picture of the day, updated daily, and the top 30 screenshots too.
Updates: 
Series 2