Using the Arachnomorph (the little spider-like guy you can transform into), you can go out-of-bounds in almost any room that has a crawlspace or breakable blocks that allow you to create your own crawlspace by shooting the lowest one.

Standing next to the crawlspace, press Down+Button to turn into the Arachnomorph and immediately press Up and slightly to the direction of the wall. If done correctly, you will unmorph and slowly rise up into the wall/ceiling, where you can then move freely.

You can get on top of the room you are currently in, which allows for easier (and in some cases maybe faster) traversal. You can also fall off of the side of the room, which doesn't scroll the screen but depending on where you fall, you might uncover map spaces far below your actual position, which could be helpful to find out about the map and which ways to take.

It's not clear yet if this glitch allows for any sequence breaks or skipped items, as it seems you can only actually get to the directly connected rooms. This may allow you to climb up a very high room without having any high jump items, as you slowly move upwards inside the walls, but it's unlikely that allows you to skip high jump items alltogether as there are likely to be more rooms where high jump is required without there being any crawl spaces to abuse the glitch.