I've done that before, it hurt for a couple of days, then just went away and was good as new again. But yeah, it hurt like a motherfucker when I first did it and the next day. Made me sadfaced all over.
The ABSOLUTE WORST though was when my girlfriend was giving me a handjob in the theatre. I didn't really feel the fact that my zipper was rubbing the side everytime she slid down on it. So, afterwards she gives me head, the like two hours later, then the next day - and at that time she looked at me wierd and said she tasted blood. I was like "Wtf, where is the blood coming from, am I like a vampire?" I had no clue and she went on, but the second she touched it again I knew where the fuck it came from. Zipper shaved off most of skin, and then the head 3x over the next 12 hours just kinda rubbed it all off. Then I felt it, then I hated life, then I didn't get head for 3 days while it healed up. ALL AROUND BAD SITUATION, I URGE YOU TO WEAR SWEAT PANTS OR A BUTTON FLY FROM NOW ON!