Internship and residency are jobs, as a previous poster said. That said, many reasons to put someone on probation really don't have anything to do with "learning" per se, and a lot more to do with unprofessional behaviour (not showing up, showing up late, leaving early without signing out, calling in sick too frequently, being unwilling to suck one up for the team.)
On the medicine side, not answering call pages, being lazy and not seeing that patient with the BP 70/50 or the one with chest pain or not going to that code can also get you in trouble.
And remember, impairment/substance abuse are a fast way out of any employment.
From what I have seen, as a student and now intern at my base hospital:
1) It's hard to get a failing eval (you really need to work at it)
2) The hospital will work with you, if you just cooperate with recommendations on how to improve.
3) The hospital needs housestaff, but not if they kill patients. NO housestaff is that important.
4) If you are having problems, you need to go to the residency director or the DME. They can help you, give you a little breathing room, etc. But only if you act like an adult.