In my test, failed motion occurs by mashing single-cast-spell key while holding movement key.
I do not test all, but lightning blast, fireball, rip blood are so.
Also, as far as I can see, shurikens, riposite, elemental nova, manastrike have same trouble.
Maybe, I guess many single attack/cast skill have same problems.
…Or simply, lag spike?
Edit:proofreading sentence