If you select something with the marque tool, then make that thing not exist, and then try to paste the original selection into a new document—the selection is lost.
Example process:
- Make selection
- Flatten image
- Copy selection
- Undo, to unflatten image (in case you accidently save-and-close)
- Make new document. Paste. Be annoyed for the millionth time, enough to write in your Tumblr
