Continuity mistake: When Mason runs into Joy at the park, a dog in the background drastically changes positions from shot to shot.
Continuity mistake: When Joy goes to work after moving in with Jack, she is in a blue dress. When she is at work, her dress is grey - the same one she had on at work at the beginning of the movie, before the break up. But when they meet later at the counselor's, she is back in blue. (Not to be confused with the red jacket she is wearing while she is working.)
Continuity mistake: At the party, when Joy rips Jack's name tag that says, "Hello I'm Joy's Bitch" and puts it on the other woman's dress, it moves from diagonal, to straight, to diagonal, to straight on the back of the dress.
Continuity mistake: After Joy shows her boss her wine bottle/knife trick, she pours a glass of wine, and when the shot changes the bubbles disappear instantly.
Continuity mistake: In the scene where Joy quits her job, you see she is wearing the ring she just gave to Jack in the scene before.
Continuity mistake: When Joy is curling her hair in the bathroom, Jack is standing at the door waiting to use it, and there's a file cabinet with wheels which disappears when Joy comes out.