Plot hole: Throughout the show it is reported several times that Billy and Ally met when they were both eight years old in a park by sniffing each other's bottoms. However, when Billy is buried in season 3 his tombstone reads, 1968-2000, making him 32 years old. Ally is only 29 and does not turn 30 until later in that season which would at the very least, put them 2 years apart. (00:41:15)
The Ex-Files - S1-E5
Continuity mistake: When Dharma is visiting her uncle (a dentist) and she's in the dentist chair, she has a little spot of water on her bib, the shot moves, then it moves again and the spot isn't there anymore.
Continuity mistake: When Sheila Carlisle has hanged the dead head up on the wall of Eugene's office, they are all looking at it and the eyes are looking straight in to the room. Later in the episode, the eyes are looking more to the right.