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.
Trivia: The name of the Mexican guy who found the Spear of Destiny is Manuel. (It's revealed in the opening scene, when his fellow calls after him in Spanish.) Manuel is the Spanish form if Immanuel, the name of the Messiah according to the Book of Isaiah. Occult interpretations of the Bible commonly claim that Immanuel will be the name of Antichrist.
John Constantine: When I was a kid, I could see things. Things humans aren't supposed to see. Things you shouldn't have to see. My parents were normal. They did what most people would do. They made it worse. You think you're crazy long enough, you find a way out.
Angela Dodson: You tried to kill yourself.
John Constantine: I didn't "try" anything.
John Constantine: So when a half-breed breaks the rules, I deport their sorry ass straight back to hell. I don't get them all but...I'm hoping to get enough to insure my retirement.
Angela Dodson: I don't understand.
John Constantine: I'm a suicide, Angela. When I die the rules say I got just one place to go.
Angela Dodson: You're trying to buy your way into heaven.
John Constantine: What would you do if you were sentenced to a prison where half the inmates were put there by you?