Continuity: When Ashley moves in with her two friends she breaks their bathroom mirror with a hairdryer. Later on in the movie, in the scene where her two friends are giving her a pep talk about luck, we see the same bathroom mirror in the background, which is now perfectly fine. Right after that, Ashley goes into the bathroom and looks at herself in the mirror, which has suddenly gone back to being broken.
Great sites
Mistakes
When Ashley is in the laundry room, while she waits by the washer for her clothes to be washed, you can see a crew member reflected on the garbage bin. See more...
Just My Luck mistakes
Continuity: When Antonio drops off the dress at Lindsay Lohan's apartment, they say "Bye" and he is a few feet down the hall and waves at her from his left. In the very next shot, he's now standing right next to the door and it's his right side we now see.
Continuity: Near the beginning when Ashley is waiting for the lift at her workplace, pay attention to the magazine she's carrying. Later, when Ashley gets off the lift after getting a date with David, she's carrying a different magazine.