^ No, benzos lessen the symptoms of the 'emergence reaction' (coming out of the k-hole), that's why they're generally given in combination with ketamine for anaesthesia for kids (also why tiletamine, used in vet. med., comes combined with a benzo in soln.). As the 'emergence reaction' is essentially the psychedelic activity people are looking for, benzos reduce the psychedelic activity; they also add to the amnesia that occurs with ketamine - combine the two and you'd chance to remember fuck all.
As for the needle being left in, I think it's occurred in people who tend to draw back and flush the syringe with blood before re-injecting it to get every last bit of drug (seems to happen quite often with heroin on its own). The same thing happens with IV barbiturates (OD victims have been found dead with needle still in their arm - I've seen a forensic photo of a barb OD like that), IV barbiturates being also used as an anaesthetic agent - well an induction agent for anaesthesia