I've just been typing HTML in a browser tab online and all of a sudden had triggered insert mode (where typing overwrites the following character) using a keyboard on a Chromebook (Chrome OS) - I had no idea how to switch it off so just had to save my draft. I never knew I had this capability and can't figure out how to get it back on. The shortcut lists online offer no hints!
I think I was playing with ctrl shift and alt but no idea what other keys unfortunately...