About 30% of all the NIC's I see in my day to day work do not cope gracefully with a crossover cable. Some fail to autonegotiate, some definitely want to be forced, some just will not work with a specific brand of NIC at the other end.

Best bet is - don't rely on it. Although it can be fine for a swift hack (and I use it most of the time with a setup which works for me) you might as well buy a £20 hub.