In NetHack, between level 15 and level 19 of the Dungeons of Doom, you will get an odd message and everything will look weird. This means you have reached the Rogue level. There's nothing really special about it, just that there are only two exits from each room, and you will automatically pick up anything you walk over. A few of the symbols are different (notably % for staircases, since corpses don't exist in Rogue), but that's about it.

If you are using the "color" and "IBMgraphics" options, the game might start behaving strangely at this point. I strongly advise turning these options off upon reaching the Rogue level. This is probably a bug interacting strangely with ncurses. If you're playing xnethack, no problem.

The Rogue level is a tribute to the game from which Hack and NetHack were derived, I imagine. The level contains no treasures or monsters of any significance.