In the episode where Grace won't get out of bed because she just broke up with her fiance, Jack comes into the apartment and goes to the kitchen to talk to Will. Will says "you mole is smudged" (Jack had an eye-liner mole above his lip) He fixes the smudge. Later Will leaves the apartment and tells Jack not to disturb Grace while she's moping. Jack does anyway, and when you see him in Graces room, the mole is gone.