It does happen quite rarely i have to admit but there is some weird case where the fireball skill does not fire in the direction of the mouse and can be totally 180 degree from where you are pointing. The only way to fix the issue is move if it's not frozen for some reason or use the teleport skill which solve the issue 100% of the time so far. I do use the extra projectiles passive and runes so i fire 3 fireball at a time so this issue is very noticeable when it happen. It is not a visual bug as the fireball going the wrong way do damage the enemies that happened to be in that "wrong" direction