My post dissapeared!

No, not from this board, it disapeared from my screen at home. The following happened:
I finished typing a reply and clicked on “Post Reply”. Insted of familiar, almost blank page ‘wate, we are taking…’ on top, a page saying “This page has expired” appeared, instantly. The Back button gave me the preprevious page, not the type-in window. Perhaps, my post is stored somewhere in the Net guts, but unaccessible to me.
I wonder:

  1. Am I the first one in the world to whom it happened.
  2. Are there ways to avoid it in the future? (I’ll copy all my posts before clicking on anything now).
    3)I am just curious of what actually happend. I know that some sites (apparently, not this), have this feature as a security precaution. But then the resulting page usually says: “This page has expired to protect your accunt. Please go to our Registration page and reenter your password”.
    What happened to me here?