Visible crew/equipment: When Judge Dredd says: "Thanks for the heads up", you can see that the gun he is holding is a rubber stunt prop: you can tell because the front end is sealed shut when it should be open. Also, the barrel is thicker, certain details are missing, etc.

Dredd (2012)
1 visible crew/equipment mistake - chronological order
Directed by: Pete Travis
Starring: Lena Headey, Karl Urban, Olivia Thirlby, Andile Mngadi, Porteus Xandau, Rachel Wood
Revealing mistake: When Dredd throws Kay onto the table, Kay is supposedly handcuffed, but his hands spread apart way further than they should have.
Judge Dredd: The perps were uncooperative.
Question: Why doesn't Kay kill Anderson after he grabs her and takes the elevator up to Ma-Ma? Or at least, why doesn't Ma-Ma kill her when Kay comes in with her? They were intent on killing the Judges and didn't care about the consequences. So what was Ma-Ma hoping to accomplish having her as a hostage that she couldn't try to accomplish without one? At this point, she already had the 4 crooked Judges on their way.
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Answer: Kay wanted to punish her for what she did to him earlier. Ma-Ma wanted leverage against Dredd should he get past her Judges.
MasterOfAll