Continuity mistake: In the opening scenes, Grace has red nail polish on. Later in the same scene, it has vanished as she shakes Mrs. McFarland's hand. (01:03:20 - 01:06:15)
Continuity mistake: When Karen is sorting out her clothes at her work desk, the black sweater hanging over the desk lamp disappears then reappears. (00:26:40)
Continuity mistake: When Grace finally decides to move into Will's apartment near the end, we see Will eating a tub of ice cream. Between shots the ice cream tub lid jumps from being on the table behind Will, to suddenly sitting right in front of him. (00:42:00)
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.
Went to a Garden Potty - S4-E20
Continuity mistake: When Karen is talking with Norman about donating funds for his advert, and then when Jack enters, between shots during the scene Karen's pearl necklace keeps jumping in and out of her black shirt. (00:45:00)
...And the Horse He Rode in On - S5-E1
Continuity mistake: When Grace and Leo are speaking at the table in the coffee shop, the muffin on the table keeps changing position, and Grace's hand keep jumping on and off of it between shots. The bites taken from it also differ.
Lows in the Mid-Eighties (1) - S3-E8
Continuity mistake: When Ellen says to Grace "Okay, what do you mean Will and You have never done it?" the red plastic cup she holds swaps hand three times between shots. (00:45:25)
...And the Horse He Rode in On - S5-E1
Continuity mistake: While in the coffee shop, Leo writes Grace a prescription. As she is reading the piece of paper, Leo puts the prescription pad back into his shirt pocket. In the next shot the pad is on the table in front of him.
Continuity mistake: After Karen takes her black top off and throws it down on the table, whilst talking to Jack as they first meet, in following shots the top changes position a few times. (00:37:40)
The Unsinkable Mommy Adler - S1-E13
Continuity mistake: When Grace is reading through her diaries, through all following shots in the scene the position / location of the yellow, furry diary on the floor keeps changing. (00:37:40)
Continuity mistake: In the poker scene at the beginning of the pilot we witness Will, Jack, Rob, and others playing cards. Shortly after Jack asks for "four cards please" watch the bottom left of the screen as a hand (presumably Will's) deals Jack a single card. In the next shot Will appears to have not dealt Jack any cards yet and when he does he gives Jack five cards, add that to the single card he gave in the previous shot and the Ace Jack kept from his original hand and we get a total of seven cards. Two more than Jack should have had if the hand was dealt correctly.
Continuity mistake: When Grace enters Will's office, while he is spinning around on his new office chair, she goes and puts her black purse down on the desk. In the next shot it is positioned differently. (00:04:30)
Continuity mistake: After Will and Grace make Larry drop the birthday cake, there is a black woman standing behind then, an a green shirt. As Will and Grace then start to shout at each other, the woman walks away three times between shots.
Continuity mistake: After Kevin throws the contract paper down onto Will's desk, the items on Will's desk, and the contract itself, change position a few times in the following shots. (00:27:05)
Continuity mistake: When Will and Grace are talking to the nun about buying the car off of her, the amount of Burberry scarf showing out of Will's coat is inconsistent with each shot change.
Continuity mistake: After Jack and Karen have locked Will and Grace in the bouncy castle, Grace throws her black purse down into the corner, then her and Will start talking. During their conversation the position of the purse keeps changing.
Answer: The director, James Burrows.