Low Level Bit Hacks You Absolutely Must Know
I was asked #1 "Check if the integer is even or odd" in an interview once. Not worded quite that way but that was the answer they were looking for.12 years later not a single person I have hired could correctly answer that question[1] and most of them (almost all) have been great productive software developers. So "absolutely must know" is entirely dependent on the type of software you write.[1] I don't ask it in the interview, I ask it after they are hired and have been with us a few weeks just for fun.