residency process...

how does this work? what happens if a student lists every choice in a match for a competitive field, but doesn't get into any residency program for that field? does she just get placed in another program, but one that still maximizes the tradeoff between what she wants and what is available, or is she just sort of screwed and placed into one with the most slots available? what kind of filtering process goes on beforehand to make these match lists, i.e. i'm assuming a med school doesn't let students at the bottom of their class try to match for neurosurgery..