I found the problem with "Post Local Storage" extension.
The back button is in my mouse so it's easy for me to press it.
It's rare but it happened to me several times.
"Post Local Storage" saves every 10sec the posted text according to url.
See what happened (with Firefox, I haven't used another browser) ... just to have it in mind.
https://youtu.be/TYD-JPNyI0I