It's easy to do a lot of things by accident IE: Using a scalar in ".multiply()" method, or taking the dot product of a scalar instead of another vector.
When this happens, these functions usually return NaN, and continue on without issue, except that you now have a NaN floating about that you have to figure out where it came from. It would be nice if there was at least some console warnings letting you know that you screwed something up instead of trying to work your way backwards.