Punching someone in the back of the head may lead to serious brain damage and even death.

Donkey punching involves the male punching the sexual partner in the back of the head during anal or vaginal sex prior to orgasm, to provoke a shock causing the vaginal or anal muscles to contract around the male's penis.

The move can be prosecuted as sexual assault and even rape in the case of surprise anal penetration.
-"YES YES"
<Donkey Punch>
-"AAAAARGH!!!"
-"Ooooh, that feels so good!!! Isn't it love? ...Love? hello? Are you okay? .... aaaaaargh!!!! Love?? Wake up!!!! Aaaaaargh!!!"
Source: Harrythepotter, homelandth, Sep 1, 2005