I don't recall the exact question, but I recall they were only looking for the answer that used bit manipulation. Which was not necessarily intuitive in the context.
Hm, perhaps counting number of bits set to 1 in a multi-byte array?
no, bit wise is logical there. I'm thinking it was duplicates or primes or some such.
I know a common one is how to switch the values of two variables without using a temporary third variable. One way (method 2 here [1]) is to use repeated bitwise XORs.