Continuity mistake: When Constantine manages 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.
Factual error: Rachel Weisz fires approximately 29 shots from her semi-automatic handgun without reloading - vastly more shots than her gun can hold.
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.
Continuity mistake: In the end when Constantine slits his wrists there is a shot where you see the pools of blood from his wrists. The scene changes for a few minutes and there is another shot where the blood is visible and the pools are smaller than before. If anything they should have been bigger.