The "proper" Hayes AT drop-back-to-command-mode sequence was +++ followed by a long pause (1 second or so). Any characters sent during that pause time would invalidate the attempt to switch to command mode. So theoretically this shouldn't happen. However, lots of cheaper modems seemed to not check for the pause, so sending +++ATH0 would make them switch to command mode and immediately process a Hangup command ... not pretty :-).
Specfically, that pause was the part of the Hayes AT command-set that was patented. Therefore lots of manufacturers who didn't want to licence the patent but wanted to have 'Hayes AT compatible' on the box just didn't bother implementing the timeout-cancellation bit.
