Near the end of the movie, when Thorn is speaking on the police call box phone, he is waiting to speak to his supervisor Chief Hatcher. While waiting he calls and is talking to Shirl. The police operator cuts in saying "I have Lieutenant Hatcher now" incorrectly identifying his rank.
When Thorn enters the apartment with the women the strap from his shoulder holster which goes around his right shoulder is in a figure 8. When he steps into the bedroom the strap is going straight around his shoulder and untwisted.
Thorn and Roth share a small apartment in a tenement stuffed with people, many of whom live in the hallways and stairwells. When Roth makes the stew, there's no apparent reaction by anyone to the odor of what must have been the first meal cooked inside the building in many years.
Tab Fielding twice uses a pistol with a silencer attached. The pistol is a .38 caliber, Smith & Wesson Police Special - a revolver. Silencers do not work on a revolver; the gap between the cylinder and the barrel will release the sound of the gunshot.
When Thorn is in the Soylent Green factory, he knocks two factory employees off the catwalk. A stunt mattress can clearly be seen sticking out from underneath the conveyor belt where the second one lands.
When Thorn is talking to Sol in the "suicide facility", at one point the inter-phone starts malfunctioning and the "speaking permitted" light starts to blink. If you look at the cable coming out from the lower-left corner of the device, you'll notice that they are obviously alternating two still frames to achieve the blinking effect, as it jumps erratically from one position to another.
When Thorn is running to the euthanasia building to try and stop Sol, he is limping from just having been shot in the leg. He gets there too late, so he follows Sol's body as it is disposed of. He goes through a long sequence in which he is repeatedly required to dart, scurry and jump, which he accomplishes with nimbleness and no sign of the leg injury. Later, as he is fleeing gunmen in the running shootout, he is limping again.
When Thorn is running towards the "going home" building, his head disappears into the matte just before the shot cuts. Later in the same scene, as he sees Sol "going home", part of his arm disappears behind the orange wall seen through the window.