had to do with my collider quadtree (lol). non-static colliders like the player each frame find which areas they overlap and insert themselves into the associated collider list(s). when they get too close to the edge they were looking for collider lists that didn't exist (outside the room), which by game maker logic means they were looking for list "0"... which was an actual list in use, and happened to be the list of level geometry in the level editor (LOL). so if the player stood near the level bounds and then went back to the level editor, the geometry list would be filled with player object id's which are a totally incompatible data type with the level geometry stuff, hence crash as soon as it tries to do things to it.

just had to make sure they only insert themselves into actual quadtree lists.