Continuity mistake: During Wikus' congratulatory/home-coming party, after he throws up and falls to the floor, he falls on his right-hand side. But in the next shot, he hits the floor on his left-hand side. With benefit of the doubt, he might have rotated/spun during the fall, but withstanding the height of the table and that he didn't seem to have been rotating a moment prior to hitting the floor in the next shot, it's unlikely. (00:34:50)
Continuity mistake: When Wikus gets into the Prawn Exo-suit, a head brace looking object clamps onto his face, causing pain. It causes blood to trickle down by his left eye. In later shots, the blood is completely gone. There is no trace of the blood left meaning that it wasn't wiped off while the camera was off of him. (01:24:40)
Continuity mistake: Near the end of the movie, when Christopher is pulled from the armored car and questioned about his plan, his arms are tied behind his back. A little later, when he is kneeling and the order is given to kill him, his arms are still tied. When Dinkus returns, Christopher is still on his knees, even after a soldier to his left explodes. A moment later, Dinkus arrives in front of him and Christopher is still on his knees, but now his arms are free.
Continuity mistake: A news announcer mentions that the suspect, Wikus, has been on the run for a week but moments later the screen shows "72 hours after exposure" meaning he's been on the run for 3 days at most.
Answer: This isn't actually remotely uncommon. Trailers are almost invariably released before the final edit for a movie is locked in place, so it's not at all unusual for shots in the trailer to be from scenes that ultimately don't make it into the finished product, or to be from alternate takes of scenes that do appear. As to why this particular scene was rejected, who knows, most likely was just for pacing, or they might simply have not liked some aspect of the acting or whatever.
Tailkinker