Dev Door

A Dev Door (or Developer Door) is a level creator resource in the Super Mario romhacking community, and in the Super Mario Maker community intended to provide a more prompt and direct access to sections of a level of considerable length that are intended to be reached through the regular route in the final product. Mainly used during the developing stages of a level, Dev doors allow the editor to reach a part of the level that would otherwise take a long time to reach if they play the level from the very start point.

Dev doors became a necessity in Super Mario Maker in particular, as the game only allows for two checkpoints per level, while doors are unlimited. Ideally, dev doors should be removed, or hidden at the very least, before the level is published, and the presence of a dev door in a published level indicates a lack of oversight and polish by the level creator.

Huh, this creator left a dev door on his level... you gotta pay attention!