Out of depth
Most roguelikes generate monsters, traps, and treasure on a level based on the depth of that level in the dungeon (for traditional dungeon crawl roguelikes, however the basic concept applies to non-traditional settings as well).
Many roguelikes will occasionally generate out of depth monsters and treasure that would normally only appear much deeper in the dungeon. This spices up the game with some exceptionally dangerous encounters that the player cannot plough through directly, but rather must exercise caution. The degree to which out-of-depthness is allowed varies considerably. Modern Angband is fairly cautious, especially when it comes to monsters, while Linley's Dungeon Crawl allows very out-of-depth monsters to appear frequently enough to be notorious for it. Generally, vaults allow for more extreme out-of-depth creatures and treasures.