The black mark is a strong argument against accepting a prospective academic. They've done something extreme, like ignoring the advice of the natives and getting incredibly drunk, or saying that they had just finished writing an R01 grant and hated it and were never doing that again (actual hearsay of a faculty job talk). These are the clear situations where the prospective has just shot himself in the foot, and would to do some fast magic to salvage the job opportunity.
Then there are the gray marks. Nothing obviously bad, no story that will be repeated with relish for the remainder of the department's existence, but something that instills doubt about an otherwise unremarkable person.
As a third-hand story. One of the prospective students had to catch a shuttle to the airport at 3, 3:30 in the morning. The shuttle stop (a local hotel) was too far to walk. The host offered to call the prospective student a cab, setting everything up the night before. But, no, what if the cab didn't come. The host should drive the student to the shuttle stop, at 3 in the morning. The prospective does not apologize for the inconvenience; it is the host's responsibility. To add insult to injury (sleep deprivation should qualify as injury), they nearly missed the shuttle because the prospective hadn't finished packing, so the host worried she would be called upon to drive the student all the way to the airport (a 45 minute drive or more).
It's hard to tell how bad this is. I have it in the retelling that there was a sense of "entitlement" on the prospective's part. I would never have asked a host to drive me somewhere even at six in the morning, but then I was never comfortable asking my hosts for a glass of juice in the morning. The general consensus is that this was a bit extreme.
Perhaps ordinarily this wouldn't have been enough to count against the student, but in this case my advisor is purportedly looking for students who can work independently, since she already has four of us making demands on her time. Here we have a prospective that isn't capable of taking a taxi for lack of hand-holding, and seems to feel that sense of entitlement (not my words) on what should happen. Three days of unremarkable interactions were so easily negated by one anecdote.
It remains to be seen whether my advisor, and the rest of the faculty, would have the same reaction to this story (assuming the host passed on the perceived problem). The host isn't the kind to hold a grudge (obviously, she wasn't the kind to refuse going out of her way, where I would probably have said "I'm calling you a cab; take it or leave it"). But if the prospective is admitted and comes to the department, she's going to start out with a bit of a negative balance; even a mildly bad impression will leave quite a bit to overcome.