I often keep Half-Life running in the background while I make small changes and quickly recompile maps. I then restart the map through the console and test again. I can verify that this is a bad way to test trigger_changelevels. Even though it's all perfectly set up, I ended up on world coordinate 0,0,0. You really have to restart Half-Life completely if you make any changes to the changelevel triggers.
In SC the black_hidden texture is not drawn. It's also hidden.