Normally opening a scene in the editor prompts you to first save any changes to the currently loaded scene. But there’s a path, only mildly convoluted, that skips this check. Load a scene, edit it, and then play the scene without saving. If in the process of playing the scene your game engine loads a new scene, AND THEN you try to open a different scene in the editor while still in play mode, the unsaved changes check is skipped.
I can assure you I experienced this first hand, and lost an hour’s work because of it. Ouch.