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 Grace is preparing the food for the thanksgiving dinner, she unwraps the plastic wrapping from the bowl of cranberries, and scrunches it into a ball. In the next shot she holds it and it is unwrapped and not a ball anymore. (01:10:25)
Continuity mistake: When Jack is lying on the table, with the huge bowl on his face, the position of the boxes behind him on the table changes a few times. In one shot they're half hanging off of the table, in the next shot they're fully on the table etc. (01:08:15)
Continuity mistake: After Will tells Jack that he knows that he hasn't told his mum he's gay, Jack backs into the patio doors, and the blind unrolls itself. In following shots it alternates from rolled up / down. (01:07:30)
Visible crew/equipment: After Will and Jack come back from the patio area, into the thanksgiving dinner area, the grey boom mike can be seen reflected in the frosted glass panel above the living room door. (01:18:20)
Continuity mistake: During the thanksgiving dinner, Will is mixing salad. He then leaves the mixing spoons in the bowl. In the next shot they've vanished, and then reappear later on in the scene, back in the bowl. (01:14:00 - 01:15:35)
Plot hole: Will and Jack are out on the patio talking about the first time they met at a party (implying in college) and how Jack convinced Will to come out, but in S1E6 Jack talks about Will's favorite book in elementary school. I am re-watching the show now, and I am sure they have a conversation about hanging out as kids. Makes no sense. Time 1 is season 1 and time 2 is for Season 2. (00:06:33 - 00:13:10)
Suggested correction: I just re-watched the scene you referenced from Season 1. Jack is talking to Grace and is throwing out random Will trivia. He never said he knew Will in elementary school, just that he knows what Will's favorite book was when he was in elementary school. There is no plot hole with this statement.