Continuity mistake: When Wynn is talking to Al about the mission to blow up the plant, Wynn puts his cigarette out in the ash tray in the scorpion bowl and there are 4 butts in the ash tray, but a minute later when the camera shows the 2 scorpions fighting, the ash tray has no cigarette butts in it. (00:11:19)
Continuity mistake: When Spawn is fighting with the clown, who is in his big reptilian monster form, Spawn is thrown against a fence with pointed tops. He is impaled against them, but the ones right next to him are shaking as if they were made of rubber, not of metal. In the next shot, the spikes are stiff and solid and made of metal.
Continuity mistake: When Cyan is given the clown doll at her party, it spins around and spits (or breaths) the green stuff and stops, but immediately in the next shot, the head has made a 180 Degree spin, which is impossible in that time.
Factual error: In scenes where Spawn has the ammo belts over his chest (at the banquet and also before getting on the motorcycle), the rounds are of some very high caliber weapon. However, the only gun(s) Spawn carries is a P-90 submachine gun, which uses magazine-fed 5.7mm ammo. What are the ammo belts for if he isn't carrying a weapon that can use them?
Revealing mistake: During the final battle in hell, when the army of Spawns rise up, there are some small errors to look for. First, during a few shots where the army is closer to camera, you can see the CGI army becoming "pixely." Also, you can see at least one of the Spawn floating in the air, not standing on the platform with the rest of him. Problems with the CGI rendering/animation.
Plot hole: Al Simmons' reasoning for wanting to quit A-6 is because he learns his rocket attack in his assassination mission in the beginning of the film yielded civilian casualties. This makes no sense, as he blatantly gunned down several innocent men in the air control tower just moments before.
Other mistake: Towards the beginning when someone is looking at a newspaper about the airliner being blown up, there are nonsense articles on the paper and pictures of people that don't match the names under them. With some white guy's picture over the name Michael Jordan on the left of the page, and then on the right a picture of a white woman with the name Will Smith under it.