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.
Plot hole: In the first episode, Richard Fish invites Ally to join his law firm, saying that he has only recently set it up, yet in the Christmas episode of series one, Elaine says that singing at the party is an annual tradition in the firm and also that they have had at least three Christmas parties. That much time has not passed since Ally joined.