What is something that was explained to you wrong such that you could never understand it, and you received a clarifying explanation that finally made sense?

When doing a convolution of two curves in time, you flip one of them backwards.

Our shit statistics teacher made it so complicated.

And then one day, next semester, in a lab for signals class, the TA casually said "flip one, so they both start at zero seconds" and half the class started convulsing as an entire semester worth of misunderstood math magically snapped into place.

Lots of engineering teachers, have no business being teachers.