I must say it happened more than once for me in the channel editbox, but never in the script editor, so if you find the shortcut for the editbox you're golden. Both are richedit control, it's weird.

I once suggested for the script editor to indicate when we are in insert mode, which is not visible just like that, right to left is pretty visible, so perhaps the shortcut should be noted in the help file or something rather than a button which would not be used at all for most.

Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net