You may already know this, but if you put a solid block amongst the top row of bottom edge of the screen tiles in Vorticons, Keen can stand on it. However, the screen will not scroll down to reveal it because on either side of it are edge of the screen tiles.
This allows levels where Keen can stand on what appears to be thin air over a bottomless pit. Could be useful for providing a way to secret areas.
Do you have any more information (quirks, etc.) when this technique is used?
It's used in Keen 4 at the end of Lifewater Oasis. A number of jawbreakers and Mimrocks are hidden below the screen. Also, in Keen 6, it's used at the beginning of the last level to hide the door to the secret area above.
If there are visual clues to cue the observant player into the safety of the pit, then I suppose it is okay. (CheesyDave did so in his Zelda II mod, with the winking pillars)