exactly - naturally this was not done on purpose. all it takes is for one or a combination of the following
1 - feet are not in exact allignment
2 - one leg is tight (could be any muscle) and exerts less pressure to the ground than the other leg
3 - foot slips
4 - towel slides on bench marginally (i have seen this happen) Most gyms these days u have to use a towel.
Naturally, these dont happen often, but can happen. And not all the time anything even happens. Its just a case of something too look out for to prevent possible injury.