Continuity mistake: After Will opens his present from Grace, he puts the black "Sharper Image" bag down on the table. In following shots, the position of the bag has changed, it rotates and changes from open to closed, and vice versa.
Continuity mistake: When Will and Grace exchange gifts to each other, Grace unwraps hers. She puts the box and wrapping down on the table. In following shots the face of the box showing has changed and the wrapping has changed position too.
Continuity mistake: Grace walks into her office to see that she has a vase of red roses from Leo. During her conversation with Karen about Leo, and during their kiss, the position of the roses sticking out at the top of the bunch changes.