Well, there was this one time when we were playing class basketball in high school. I was hit hard on the side of my nose (I brushed it off as a foul) until I was shooting the second free throw for that foul... the referee called an injury timeout for our team since my nose started a pretty nasty bleeding. A fair lot of blood was coming out of my nostrils. Wasn't a good experience - after the adrenaline wore off my nose was stinging and I keep smelling blood for 3 days.
But hentai... nah. Not yet. But I won't like it if it does happen.
no, never get any nosebleed from watching H-stuff. Last time I get a "normal" nosebleed when i get my junior high final math exam. after I finished and get out, my head feels dizzy and my nose suddenly bleed.