When Will tosses all the ties out of his cupboard, the tie which lands on the end of his bed keeps changing.
While Jack is reading his speech, right before realizing he is actually 30 years old, Debrah Messing (Grace) sneezes. She turns away so it is less visible on screen but it is still clearly a sneeze that was left in the edit.
When Sam is about to leave Jack's party, Grace tells him that it took him "5 years to get here" to where he and Will is "only 15 feet apart". She goes on to say that "that's like 3 feet a year" and made a joke about Merchant Ivory films. However, she divided the remaining distance (instead of distance traveled) by the time it took. This makes the subsequent joke illogical.