Continuity mistake: After arriving back in Metropolis, Superman first tries to defeat Doomsday by simply blowing on him to freeze him. You can see that he is standing almost right next to the tank he just sat down when he is in the process of freezing Doomsday. But then once Doomsday has stopped moving and is frozen, the shot cuts to a different angle and suddenly he and Doomsday are much further away from the tank, and somehow trees behind Superman are half covered in ice as well. (00:18:20)
Continuity mistake: Doomsday throws a tank at a group of soldiers, and Superman swoops in and catches it just in time. He slowly sets the tank down and there is a black star on the back of it. It cuts to Doomsday sprinting at Superman, then cuts back to Superman as he is about to inhale and blow at Doomsday to freeze him. Now the black star has vanished. The damage and shape on the tank's cannon also slightly changes, the wheels on the tank change as well as the tracks. And the red van next to the tank is suddenly further back. (00:18:00)
Continuity mistake: Doomsday charges a tank and picks it up, throwing it at the tanks behind it. You can see as Doomsday picks up the tank, they are in the middle of a straight stretch of road with no other roads intersecting close by. But as the tank lifts into the air, the camera cuts to a side angle and suddenly the other tanks are sitting right in the middle of an intersection. And the buildings around them also change a lot. (00:17:30)
Continuity mistake: After the soldier in the tank fires his .50 cal at Doomsday, the tank turns its cannon at him and fires. Behind the tanks you can see the guard rails of the bridge extend far into the city, showing they are still high up. But then when Doomsday charges the tanks the railing is shown stopping as they are at street level now. Also the city behind them changes up. (00:17:25)
Continuity mistake: After Doomsday crushes the skull of a soldier, a tank rolls onto camera and another soldier begins firing the .50 cal gun at the creature. It then cuts to a close up of the soldier from his front and the position of his left hand changes. From being on the side of the gun, to being on the top of it. (00:17:10)
Continuity mistake: When Doomsday is on the bridge in Metropolis, he causes lots of destruction there with fire and death. Moments later, Doomsday charges at a tank and punches the missile that it fires at him, blowing it up. The camera pulls back and shows the bridge behind him, with no fire or smoke or anything that was there previously. (00:17:20)
Continuity mistake: Once Doomsday makes it to the surface, he spots a deer and it is shown from over his shoulder. It cuts to a view of the deer from Doomsday's eyes and suddenly the deer has moved over some, and the rocks and grass around it have changed. (00:11:30)
Continuity mistake: The robot at the Fortress of Solitude has black arms, including most of the shoulders except where the pivot joint is visible. But for one shot when Superman is saying he wants to be more than Earth's resident strong man, the robot's shoulder is solid silver color like its torso. (00:07:55)
Continuity mistake: When the laser drill is first shown at Project Applecore, you can see a protective bumper like thing on the front of it colored black and yellow in the caution stripes. Two scenes later when it is shown again as the workers decide to breach the spacecraft, the bar has changed. The lines being thinner and more of them. It also appears to be a perfect rectangle the second time whereas it was lopsided the first time. (00:05:20 - 00:08:50)
Continuity mistake: After Superman fails his experiment to cure cancer, Lois walks up wearing just a towel and holding a smaller towel, drying off from a warm bath. It cuts to a different angle and suddenly the parts of her hair draped over the front of her shoulders down her chest are shorter, and the ends of the hair go from a single point to being forked. Her grip on the smaller towel also changes, her hands closer together, and her mouth is suddenly open when the shot changes. (00:08:10)
Continuity mistake: After Tess walks in, Lex turns and begins pressing buttons on a small hand held device. There are three green lights at the top. When the angle changes, suddenly the lights are smaller and his grip on the device changes. (00:06:45)
Continuity mistake: When the laser drill at Project Applecore is first shown, it has open wheels. But once the workers discover Doomsday's ship, the laser drill is seen backing up, and suddenly it has fenders. (00:05:25 - 00:06:20)
Continuity mistake: In Project AppleCore, once the forklift goes by carrying a load, you see in the distance where Dr.Murphy is standing still looking at a wall. It cuts to a close up and suddenly she is walking from the same spot she was standing. (00:05:30)
Continuity mistake: After the workers at Project Applecore discover Doomsday's ship, the scene changes to Lex in his office staring out the window. It cuts to a further shot as Tess walks, and suddenly Lex has moved over some to the left closer to the edge of the window pane. (00:06:30)
Continuity mistake: Behind Clark when Perry asks him about his flight, the elevator door is seen behind him with its green design lines all over it. Then after talking to Lois, he gets in the elevator and the door closes, and now the diagonal green lines towards the top have become much thinner. (00:04:00 - 00:04:50)
Continuity mistake: At the Daily Planet, Clark picks up a picture of his mother and smiles at it before setting it back down. Between shots, the picture changes slightly, Martha's image becoming larger compared to her background. Most noticeably the window in the barn is closer to her head as well as the bush by her neck. (00:04:30)
Continuity mistake: When Lois and Perry are arguing in his office in the beginning, you can see his many plaques on his wall. Moments later when Perry is standing in his office door way talking to Clark, you can see the same wall and corner behind him. Now there are fewer plaques and they have moved around, most noticeably the round orange one now on the other side of the corner. Also the desk with books on it under the round orange plaque vanishes. (00:03:30 - 00:04:05)
Continuity mistake: Perry asks Clark if he should be packing for his flight and you can see the cubicle directly to Clark's right there are some brown papers of some sort on the wall of the cubicle. Then a moment later in a close up of Clark, the brown items have become a wall calendar. (00:04:05)
Continuity mistake: When Clark goes to pick up the blue lunch box next to his mom's picture, it shows his computer tower at his cubicle. On it above the red light, are two rows of 4 rectangles. Then it changes angles and now the first row of rectangles is thinner and more square shaped. Also between shots the extended gray space between the darker space where the rectangles are and the red light becomes thicker. (00:04:20)
Continuity mistake: After Dark Superman knocks Superman into his memorial stone, Dark Superman (the one dressed in blue) comes down and hovers over Superman (dressed in black) and gloats. Behind him is the bulldozer. After Lois shoots Dark Superman with a Kryptonite beam and then Superman tackles him, they slide across some oil next to the same bulldozer, that now looks completely different. (01:10:20)
Chosen answer: When they call Doomsday a machine, they don't mean a literal machine. Just that he was built/designed. The comics were much the same. Doomsday was created by placing a baby on the most dangerous planet in the universe, and cloning that baby every time it died, forcing it to adapt. Doomsday eventually escaped this torture.
MasterOfAll