Lows in the Mid-Eighties (1) - S3-E8
Revealing mistake: At the party it is obvious none of the red plastic cups have drink in them. (00:45:00)
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. (00:04:20)
Revealing mistake: Grace has a present for Larry and Joe - a cement handprint decorated with twigs. She made it when she was twelve, and after a couple of decades the twigs in the cement are stick alive with healthy green leaves. (00:30:50)
Three's a Crowd, Six is a Freak Show - S3-E10
Revealing mistake: Grace's boyfriend has six toes but in closeup shots, when he takes off both socks you can clearly see only five toes on each foot, when they are specifically trying to hide one foot.
Mad Dogs and Average Men - S3-E18
Revealing mistake: When Grace eats the dog food from the frying pan, we hear the sound of it sizzling, but we see no steam coming from the food. (00:09:05)
Poker? I Don't even Like Her - S3-E19
Revealing mistake: Karen clearly doesn't suck any air from the balloon (that's why her voice doesn't go high.) (00:33:55)
Answer: The director, James Burrows.