Near the end of the first scene in the park, Jack throws a beautiful, spiraling pass. Unfortunately for right-handed Jack, the football is spinning in the wrong direction for him to have thrown it.
When Jack taps the glasses with a spoon and breaks two, he puts both of them on the top of the piano. When he goes to do his toast and says "Hi" there is only one and when he says "and who better to start things off" there are no broken glasses. Then when he says "it pains me to admit it" one reappears. Two are there when Karen takes the stage. Jack re-takes the mike and Will makes his speech there is one again. There are two when Will says "So..." after Rob jumps up and announces he isn't gay. When Leo takes the stage the whole mess seems to have gone or shifted to the right.
When Will, Grace, Leo, Jack and Karen are out in the park with the picnic, the football which Jack throws boomerangs back. The items on the picnic spread change position a few time, and also after the football smashes them.
Karen's wrap moves from off her shoulder to on her shoulder and back and forth depending on the camera angle.
The football that Jack tosses right-handed is spinning the wrong way. It is spinning as through a left-handed person threw it.