Factual error: In the scene when everyone is in the white Hummer, the vehicle is moving north on the Las Vegas Strip; on the right is Bally's and the Flamingo, with Treasure Island straight ahead in the distance. Yet a few seconds later, when Joy asks the driver to let them out, they are in front of the fountains at Bellagio, which indicates that they are now travelling south on the Strip - yet we never see the Hummer change direction or turn around.
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.)
Factual error: In the scene with Joy in her "penthouse suite" at Planet Hollywood, the view out the window has two errors: it shows the "Eiffel Tower" at Paris and, off in the distance, Mandalay Bay (the gold colored building with a white band at the top). This indicates the view is to the south, yet the "Paris" hotel/casino is north of Planet Hollywood, not south; and "THEhotel at Mandalay" is missing (it's a gold colored building to the immediately right of Mandalay Bay when looking south - I stay there often and it's a great place; it's visible at 15:46 to 15:49). (00:17:45)
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.