In my map, I've got a bunch of doors with glass in them that slide upwards into the wall. Everything compiles fine, and Hammer can't find any errors in the map, but when I play it, at certain points the doors dissapear for a few seconds so that it just looks like a floating slate of glass, only to reappear again when I look at a different angle.
Some pics:
The door, normal (sorry about the lighting). And when I look a little to the right: When I turn back, the door reappears again.
Any idea what could be causing this?