Don't know if this was posted already, but if there is an item very close to a wall, or sector (maybe even in it, but still able for the player to get it), you can go through/inside of the sector using a monitor, and you can use a ring (if respawnitemtime is 0) to go inside easily.
If you need a demonstration, I can reproduce this effect.
<strike>Test this.</strike>
Unfixed.