When Will and Grace are talking to Karen outside, as she sits on the bench, about how they'll always be a family, the people walking past them on the street behind keep changing, appears, disappearing etc. between shots.
When Will and Grace are standing outside Will's apartment, after Grace arrives back from her honeymoon, Grace is holding a handful of photo envelopes. Between shots the way she holds these envelopes changes.
After Will and Grace find Karen asleep on the top of the sofa, Karen sits down on the sofa, and puts her beer bottle on the couch next to her. When Grace comes over with her holiday photos, the bottle has gone.
After Will and Jack unwrap their breakfast Jack puts the wrapping that the Smart Start cereal box was wrapped in, on the table. In some shots during the rest of their conversation, the wrapping actually vanishes in some shots.
When Will and Jack are having breakfast, in one shot Will pours some milk into his cereal bowl, then puts the milk bottle down on the table, and it doesn't have the lid on it. In the next shot the red lid is now screwed on top.