They can be a sodding nightmare - I've had a hate campaign waged against me by a student this term, for daring to point out that her paraphrases are actually direct quotes with one or two words changed. I'm not alone either - there are numerous "problem" students with a variety of deeply inappropriate behaviour towards staff, unfortunately.
It's a pita, not because we've done something wrong but because of having to spend time and effort putting together the counter-case and doing all the paperwork. Ten years ago I didn't save all this stuff to prove that I was doing my job appropriately - today, you'd be barmy not to, frankly.
And as PP said, you quickly stop bending over backwards once you realise it's always the students you've gone over and above for who want to shaft you when they don't get the (distinction) they want.
It's probably the obvious result, though, of treating them like the consumers, and putting so much weight into trying to bribe them to give positive responses on the NSS 
Pretty fucking depressing, though, whatever the reason.
(Cheery aside: one of my supervisees has complained to the HoD because I told her that it wasn't my place, as supervisor, to identify and list out a complete bibliography for her Master's dissertation, and that this was an integral part of the task itself. I made the rookie error months ago of providing her with an introductory reading list, that she'd asked for as a foreign student from a different discipline... As I am her third - or possibly fourth - supervisor in as many months, because of her behaviour towards them, the HoD was even fiercer with her than I was 
I know it's a money-spinner, but I really think we should go back to reading personal statements, and requiring a damn good justification for admittance in the absence of a degree in the subject or a cognate discipline...)