Which number is the odd one out: 50, 9, 28, 65?

Each of 9, 28 and 65 sits exactly one above a cube: 9 is 8 + 1, 28 is 27 + 1 and 65 is 64 + 1. 50 is also one above something tidy, but that something is 49, a square rather than a cube. That near miss is the trap — the number obeys the shape of the rule while breaking the rule itself.