Character mistake: Happy determines that the nitrogen hydroxide ate through the drum because it was "steel alloy," in contrast with Walter's requirement that it be "pure steel." These terms are inaccurate, and would not be used by technical experts like Happy and Walter. Every steel is an alloy, combining at a minimum iron and carbon; steel with other alloying agents is called "alloy steel," not "steel alloy." Because steel doesn't refer to one specific substance, "pure steel" could never be used to specify a material for a given application.
Revealing mistake: Walter accesses code on the sick girl's laptop to trace the source of the virus - aside from the fact that the code comes straight up a fraction of a second after his hands even touch the keyboard, the code displayed in the black window is for a set of traffic lights, with lines like "Walk/Dont_Walk" and "Green_Arrow/Yellow_Slow." (00:08:10)
Other mistake: When Happy goes to see if the backhoe will work to finish freezing the water she says the hoses are damaged. The hoses she indicates as damaged are the ac lines and hydraulic lines that hydraulic oil flows through - she does not even look at the water hoses. (00:18:40)
Continuity mistake: Walter and Paige begin dancing and there is a lady dancing in a green dress behind them. In the next frame, that same couple is dancing across the room and then a second later next to Walter and Paige again. Walter's left hand holding Paige's in a dancing position also changes from open to closed. (00:26:05)