Single Point of Failure - S1-E2
Character mistake: In the letter informing him of his sister's condition it says "Nor have I scene" instead of "seen". I would not expect a medical professional to make that sort of error. (00:02:20)
Extinction - S4-E1
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.
Answer: A propeller sucks in from one side and pushes away from the other side.