In a level I'm developing, the player at one point has to flip a switch, and this permanently changes one section of the entire level. I didn't want to have to copy every screen over again (so that there are two versions of the level: one without the change, and one with it), so I planned on using a flag warp. That worked, but the problem came with the warp from the changed area to the main area. It didn't work at all.
I kept getting a void screen, except, oddly enough, for the first time I went through the warp (first time per game). Also, it usually works in the level editor. So I tried making a room after the nonfunctioning warp room that was filled with shifts, which would then warp the player to the correct place. That didn't work either, although again it usually works in level editor. Please help! If I can't get this to work, the level will probably take at least twice as long to make, because I'll need to copy a ton of screens.