Happens even once you reach grad school. If you are admitted and discovered to be academically unprepared, you can be asked to leave…This happened to a grad student in my department at Cal - she didn’t even make it to the MA. And there were several students who were asked to leave after receiving an MA, rather than continue on to the PhD, because they were found lacking in the necessary skills for that level of research.
So even once admitted, you still need to be pretty top notch or you can get dropped.