Quick answer: GameMaker async I/O completing after Room End firing on a different room's objects? Cancel async work on Room End; otherwise callbacks fire on destroyed/new objects.
Room transitions while download is in progress. Callback fires; finds the wrong room; corrupts state.
Cancel on Room End
http_request_cancel(req_id);Stops the in-flight request before room transition.
Or check room
Async callback checks room ID; early-return if different. Defensive.
Track per-room requests
Cleanup all room-scoped async work at Room End. Per-room cleanup ritual.
“Room End is a state change. State changes need cleanup.”
Build a per-room cleanup helper. Every async work registered; cleaned on Room End. The pattern prevents the bug class.