Steam coming from AC vents in Chevy S10?

My buddy drives an older Chevy S10 Pickup (late 90s, early 2000s). We were going to a soccer game in it and when he shut off the truck steam was coming from the AC vents. I was pretty surprised but he said, "It's fine, doesn't overheat or anything." What the hell could be wrong to make steam come from the vents? via /r/cars http://ift.tt/2tZMio8