Revealing mistake: After the opening credits there is a shot showing the house on the island where Karen and Stan live. If you look at the hanging chair, you can see that Karen is sitting in it. But in the very next shot she is in a meeting with Will, in a different outfit. This is a re-used shot from the previous season, which shouldn't be there.
Continuity mistake: When Grace tells Will that it is laundry day and that she has been wearing a peach bridesmaids dress all day, Will says, "Oh, um there's a roll of quarters in my top drawer right beside." When he says that he has a fork on his hand, when the shot changes he says, "My eye cream and my pedicure kit," suddenly the fork has disappeared.
Continuity mistake: When Grace is telling Will about his dad's affair, she mentions that he grabbed the other woman's ass when she brought him a packet of Goobers during the interval. The thing is, she sees them for the first time during the interval, and we see the conversation that follows. The mistress brings him wine, not Goobers, and he doesn't grab her ass.