If the five waves in the X wave is in the shape of a rising wedge, than it can be considered a abcde corrective structure. Also, wave failures within the X wave can look similar to a five wave.

Right count on top, wrong count in red on the bottom.

A 5 waves impulse could actually be a corrective wave in disguise. I don't know if I missed any other combinations, but you're right, wave X can't be a impulse.