It happened to me once, probably because I pulled a Something About Mary and unloaded the gun before I went out for the night.
Bad, Bad idea. The less you jerk off, the easier it is to get sir willy up when it matters.
My story has a happy ending though...that was the first time that ever happened to me, and I was embarassed as hell. So I got some Viagra from one of my friends.
Went two rounds, and was till poking the chick in her back the entire night.