A is B and B should be A, but A does not equal B.
Here's what I can think of:
Time is space, and space should be time, but time does not equal space.
Dreaming is thinking, and thinking should be dreaming, but dreaming does not equal thinking.
A real number is a rational number, and a rational number should be a real number, but real numbers does not equal rational numbers.
Sex is love, and love should be sex, but sex does not equal love.