On the back of my guitar, there's a crack right between the head and the neck. Pics later, but should I be worried that the head of my guitar might fly off and become lodged in someone's brain?
I don't think it has that amount of tension, I saw a jackson neck barely hanging on one time with strings attached. . . at worst it will fall do to the lack of tension and weight of the headstock.
Quote by paranoid joker

Metal, should kick you in the nuts, after you catch it messing around with your girlfriend.
and then make a sandwhich in your house and walk out.

you mean headstock or heel?
if its the heel of the neck, where it meets the body, it's probably just a crack in the lacquer.
if it's a crack near the headstock, you shouldn't be worried about it flying off, but you should most definitely stop ****ing playing it. DESTRING IT IMMEDIATELY. go get some gorilla glue and glue that ****er back together.