A few people have had lengthy posts disappear (when they hit submit) because their session had expired.
I usually do a Control + A (select all) and Control + C to copy the post into memory before hitting submit just in case I lose the post. Fortunately as admin the system doesn't log me out that often and I rarely if ever have to call on the copy in memory.
Today I have changed the session logout for everyone from two hours to 12 hours. Hope that helps and apologies to those of you who lost posts.