If you PIE, then eject, select something (eg. a static mesh), and hit Ctrl+C, followed by Ctrl+V, then reposses, things that get spawned in play will remain when you stop playing. Eg if your game creates explosions, they won’t appear in the play session, but when you stop, they’ll spawn in the editor viewport.
Quick video: Unreal Engine 4 - Bug Report - Copying an actor during PIE = Boom! - YouTube