This one's pretty tame compared to some of the shit posted in this thread, but I think it's still worth a mention. I actually already told this story about a week ago or so in the thread about menstruation, but no one seemed to pay it any notice.
Anyway, in about 2008 or 2009 I was having sex with my wife while she was having her period. We were doing it in the "cowgirl" position and she started leaning back, and eventually leaned too far back such that my dick popped out of her and kind of flung back towards my face. Long story short, I got a face full of menstrual blood spattered on to me -- on my chin, my lips, my nose, my eyebrows etc.
Not exactly sexy time. No bueno.