If you fail in 1st semester you can reapply as many times as you want, but have to be accepted again. If you fail 2-4 you can re-enter, do that semester over. But, you can only do that once and then you'd have to go back to 1st semester!
We don't get to fail. If you get less than required to pass a class you may get to write a supplemental exam but if you fail you're out. If you do fail out you can reapply but with competitive entry it is difficult to get back in.