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 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.
Continuity mistake: After Jack and Karen have locked Will and Grace in the bouncy castle, they talks to each other through the netting. Between each shot, Grace's purse constantly moves from on her elbow, to up on her shoulder.
Continuity mistake: When Will and Grace fight over the birthday cake which Larry holds, it results in the cake being dropped. Note the position of the cake after it is dropped to the floor. In a later shot during their conversation, it has changed.