Continuity: When John is kicking Angela out of the apartment, the cat on his lap is facing him/facing away from him between shots.
Continuity: When Constantine is talking to his doctor about his lung cancer, the x-ray light box behind the doctor is on/off between shots.
Factual errors: When John is talking on his cell phone to Beeman the line goes dead and a dial tone is heard. Cellular phones do not have dial tones.
Audio/visual unsynchronized: The kettle shown in the opening scene whistles as it boils, increasing in volume and pitch until the gas underneath it is turned off. However the kettle has a normal spout - not a whistle - and should not be whistling at all.
Continuity: Constantine traps a spider with a glass and blows some cigarette smoke into the same glass. Later, Angela lifts the glass, releasing the spider. There is considerably more smoke in the glass when the spider is released.
Continuity: John's blood surrounds his watch before Satan's appearance. Later, there is a bird's-eye view of the pools of blood and the watch is several inches away.
Continuity: When the street lights are shutting down, before the demons attack Constantine and Angela, the red traffic lights can be seen to shut down twice.
Continuity: Angela fires her gun consecutively 30 times without reloading, but her Smith and Wesson Model 6906 holds a total of 13 rounds.
Continuity: When Father Hennessy is dead and lying there, one leg is up. In the next shot, his leg is down, then back up again in the next shot.
Continuity: John gets his weapon from Gabriel and points it upwards. In the next shot he has it pointing at Gabriel.
Errors made by characters (possibly deliberate errors by the filmmakers): When in the car about to call Beeman, Constantine says that the bible in hell gives a whole new meaning to "Revelations". The last book in the bible is actually "Revelation", without the 's'; something Constantine should have known.
Revealing mistakes: When Angela is looking at the video of Isabel's suicide, she rewinds it, but the button she pushes on the far left is not the "rewind/search" button on QuickTime Player, but instead the "return to beginning".
Factual errors: The priest tells Angela that suicide is a mortal sin and therefore she cannot have a Catholic funeral. While the church does still consider it a sin, it is not done so in case of mental illness, which in modern times is every time, and in that case since she was in a mental institution is a moot point. People who commit suicide are no longer refused a Catholic funeral.
Continuity: When Constantine decides to cut his wrists so Lucifer comes for his soul, the blood spills and touches his Oris watch on the floor and time stops. On the next scene, from above, the blood pool is smaller and doesn't touch the watch. The next scene, taken from below, the watch is again in the blood pool.
Continuity: Whenever Angela draws her weapon, there is a sound of the hammer being cocked. The Smith and Wesson 6906 has a "bobbed" hammer, no "handle" to get your thumb onto easily. Cocking the weapon is difficult, especially since her thumb never goes near it. It's also policy for most police departments to fire the first shot without cocking the weapon first. In some departments, not following this policy is grounds for immediate dismissal.
Continuity: When Lucifer rids Constantine of his lung cancer he rips open his shirt but in the following scene when he goes to revive Angela his shirt is fully buttoned, devoid of any signs that it was ripped open.
Crew or equipment visible: A crew member is reflected in the water when Constantine drowns Angela in the bath tub.
Continuity: When Angela goes fully submerged in the tub for the crash course, she holds her hair back. In the next shot, her hand is not holding her hair.
Continuity: By the end of the movie when John's hair is all wet, his pony constantly changes between takes and scenes - sometimes it's hanging and sometimes it's stuck to his forehead.
Revealing mistakes: In the beginning where Constantine is exorcising the little girl, the mirror gets pulled out of the window. The line used to pull the mirror out can be seen clearly attached to it but, in the next overhead shot when it is falling toward the car, the line is clearly no longer attached.
Continuity: When Angela is vomiting after the first demon attack on her, she begins by vomiting into the corner next to the pillar on the right of the visage of Christ, but when the angle changes she's now vomiting directly in front of the visage, facing the opposite direction.