Continuity mistake: When Constantine manage to get the mirror through the window he falls on his back. To the right on the screen a chair lies on the floor beside a dresser. A few shots later when it cuts to Constantine getting up we can see the chair has changed position as its back now covers the opening of the dresser.
Factual error: When Angela is watching the security video on her laptop of her sister committing suicide, you can see she's playing it using Quicktime. She rewinds a couple of seconds by clicking on the first button of the video player. Actually, when you click this button, it takes you to the beginning of the video. The second button would have rewound it a short time.
Continuity mistake: When Constantine gets thrown into the glass paned door at the end just before he slashes his wrists, every time it cuts to the scene showing the door the broken glass left in the door is always in a different pattern.
Continuity mistake: When they are luring the demon into the mirror they have tied a chord to it. When the mirror come flying out the window we can still see the chord tied to it. But when it cuts to the mirror falling down hitting the car the chord is gone. And don't tell me the chord just went off, because it was tied quite well.