A mistake made which results in something potentially dying. It's a pretty gosh darned awful mistake if that something turns out to be you. There are two deadly mistakes which are incredibly common;
1) A woman, in the midst of her mental cycle is being somewhat emotionally unstable, and a man utters these words....."what's your problem?!" DEADLY MISTAKE!!! The obvious answer to the question will be that HE is the problem, and if he escapes with his life, all happiness will most certainly be killed for him for the next 3-5 days.
2) When getting down to some serious foreplay, the woman turns to the man and utters these words..."do I look fat?" DEADLY MISTAKE!!! There is no "safe" way for a man to answer that question! If he looks, he's judging- if he doesn't look, he's not taking the question seriously- if he says yes, she cries- if he says no, she says he's lying.... Apart from that question being the one that probably makes men want to strangle women, asking it will kill desire stone cold fucking dead. Not to mention that a "yes" response might result in someone finding the guy the next morning with his head torn off.
"She was just nagging and whining and bitching, so I asked her what her problem was!"
"Ahh, that explains the bruises. Deadly mistake, mate."
"She asked me if she looked fat, right when I was about to take her from behind!"
"Gaaah! Deadly mistake! I bet that killed it!"
"Too fucking right, I was too stressed even to wank, and she just led there muttering and texting about what a cold hearted tosser I am"