Visible crew/equipment: When Kel gets up to get more water you can see one of the cameras.
Big Gino / Jamie O. In Love - S4-E2
Other mistake: In the cafe, after Charice speaks, when Gerald says "Aloha, Gladiators on Ice", there are three places where Gerald's mouth disappears.
What Does Blue Want to Make? - S1-E15
Visible crew/equipment: A boom mic is visible when Steve starts singing "We Just Got A Letter."
First Kiss - S1-E17
Visible crew/equipment: At the end of the episode when Harvey hops away, a boom mic is visible. (00:21:55)