95. 98 had an even better one... just hitting cancel on some login boxes would let you in.

I believe that was by design: the dialog was an opportunity to authenticate with the domain. If you just wanted local access you could hit cancel. Remember Win9x was not a secure OS itself.

Pretty sure I've seen a similar trick on XP or later as well. (I learned it from someone I didn't meet until long after I last saw a 95/98/2000 machine.)