I tried to date a guy who was hot, sweet, funny, and a totally great catch, except his mouth tasted like shit. Literally shit.
I had never noticed the bad breath until after kissing him and tasting it, and then I couldn't stop smelling it every time he spoke.
I think maybe he didn't floss regularly enough, and there was rotting food in his teeth or something.
Coincidentally, I also had the same taste in my mouth for a short time after I became a vegan. I expected (and had heard) that vegans have less problems with bad breath and body odor, because the digesting meat and dairy are likely to smell spoiled or decayed.
But I think there must have been a detox action or something, because after I stopped eating animal products, for the first two or three weeks, my mouth tasted like shit, and I stunk with bad body odor.
I kept brushing and flossing and showering, and it left and was replaced with a fruity/earthy vegetable taste and smell.
Also, when I first joined a gym, I hadn't smoked for a couple of years, but the first time I went into the steam room, my pores started exuding a dirty-ashtray smell that nearly made me gag. And after about two or three trips the steam room, it went away.
Apparently it was old nicotine toxins stored up in my muscle tissue.
Since then I occasionally am in the steam room with someone who smells like that, and it's clear the person is either a smoker or it's their first time in the steamroom after quitting cigarettes.