@puzzlegirl: it just happened this one time when the board went to read only mode for a few minutes. (There’s another thread asking exactly what happened.) I think you had to happen to be composing a message exactly when that happened to see it. People also say this was the longest they’d ever seen the site get stuck in read-only mode, which might also be a factor.
Also, it occurs to me that I didn’t quite clearly state the actual problem (though you can figure it out if you know what “modal” means.) The issue is that the modal dialog box prevents you from continuing to write your post. If you click the button to reload, you lose access to your post. And, if you close the dialog, the second you try to type a few more characters, the dialog pops back up again. So it effectively stops you from composing your post until the site is back up.