A blow job that was not purposely given. The giver usually recognizes this either mid-blow or in the morning after and wonders what they were thinking giving this guy a blow job. The giver either has no interest in the reciever or actually can't stand the reciever, but somehow end up giving them a bj anyway. This normally occurs after a long night of drinking.
Ashley woke up in the morning and felt someone trying to cuddle with her. She realized then the night before she must have tripped and given him an accidental blow job.
Watch out for those trip wires, you might end up giving an accidental blow job.