I remember a punch press shearing its (rather large) bolts off and flinging them across the room because the rotation wasn't correct. It was rather dramatic...
Like beenthere says, it depends on what you are trying to run backwards.
However it certainly shouldn't hurt the motor itself to be run in a different direction.
And there was a time when I forgot to engage a magnetic clamp on a surface grinder and flung my project 40 feet through the air and embedded it into a cinder block wall.