This is for UMSL...I'm not sure how much has changed since this all applied to me....
We had proficiency exams on specific skills for different classes. If you don't pass it, you take it again but (at least when I was there), the best score you can get is a 70%. It's very uncommon to have to retake it more than once. I'm not sure what happens if you don't pass it the second time.
The "BIG" proficiency is the one at the end of 2nd year where you do a full exam to prove your competency to see patients in the clinic. Occasionally people have to retake a portion of it...occasionally someone will have to retake the whole thing (less common). If this happens, they "rehab" you (from what I hear, it's not a big deal, they just try to help you if it's something you're having a hard time with) before you repeat the exam.
I've never heard of anyone getting booted from the program for failing a proficiency, though I did know of one person who was repeatedly having problems in clinic and ended up having to leave. Really, as nerve-wracking as proficiencies are, they usually go okay.
They aren't really that bad. At PUCO (where I am now), if you don't pass the proficiency you can retake it but you won't get rescored on it. You get whatever you got the first time. But, you have to pass it in order to pass the class. It's not as bad as it sounds though, it works well and pretty much everybody passes without a problem.
Ours are on a pass/fail basis. If you fail, you get 2 more tries to pass, then I think you fail the class, then I don't know what happens. We had 3 practicals this semester and they are easy but it always makes me nervous with someone watching and timing me!