Character mistake: When Will tries to quote the law to the barman, he states that the group must be "accompanied by an adult" in order to drink alcohol with food. However, since he just served Jay, he believed Jay was an adult; and Jay is accompanying the group. Both Will and tha bartender miss this.
Continuity mistake: In Season 2 Episode 22, Model Behavior, Woody has gotten his braces off. However, in this episode, he has braces back on again.
Land of Hold/Penny Henny - S2-E17
Deliberate mistake: When Garfield brings Jon to the broken TV, the scene's animation is still unrendered. (00:01:04)
Bat Mitzvah Crashers/Super Secret Weakness - S1-E5
Continuity mistake: In "Li'l Orphan Happy" when Ben makes his mom the tuna salad, it has lettuce around it, but in the close-up the lettuce disappears. When it gets zoomed-out on, the lettuce is back.
Revealing mistake: During "Brand New Day", watch the background as Dr. Horrible appears as a giant and stomps down the city-street. As he takes his first step, a piece of concrete from building to his right falls off and hits the ground. However, if you look closely, it's obviously a quickly-made computer effect. As the piece of concrete hits the ground, it simply vanishes, and is replaced by a rather odd-looking animated "poof" of dust. The issue is the concrete piece was much bigger than the cloud of dust, so it's clear that the piece simply disappears right as the dust-poof appears.
Gary Breaks Up His Ex-Wife and Girlfriend - S1-E5
Continuity mistake: When Gary asks Vanessa if she really has a cheerleader uniform, the bag which was on her right shoulder suddenly appears on her left one.
ReTRUEnion - S1-E7
Continuity mistake: When Ryan is telling his part of what Amanda might have looked like in high school, as he put his shades to his head, his hair goes back with the glasses. When the camera cuts his hair is visibly in front of his head.
Revealing mistake: In the scene where Mitchell first visits McNair's van, Mitchell's reflection is visible in the window on the left side of the screen during the shot where he first opens the van door. The vampire lore of the show informs the viewer many, many times that vampires (such as Mitchell) and ghosts cannot be seen in reflections, pictures, or videos (their image "cannot be captured"). The reflection can be seen for roughly 20 seconds. (00:23:20)
Continuity mistake: At the start of the episode while Eli is shaving he turns around and some of the shaving cream on his face goes onto his shirt. The next shot when he leaves the bathroom the shaving cream on his shirt is gone.