I was composing a question on DBA without realising that as a new user there I would need to link my network account.
The post editor automatically saves post drafts, but when I linked my account to DBA, the question edit page reappeared, but with only the very first line of the question text present. The large majority was lost.
The tag I entered also disappeared.
(Luckily, I chose to copy the text before submitting.)