We had a power failure or a power overload (lightning?) while we were out of the house a few days ago, I'm not sure which. Several odd things happened, including an Apple power supply shutting off and having to be unplugged for a while to reset, a computer nic disappearing and reappearing after a reboot, and others... ...including the DIR-655 experiencing complete amnesia!
When I got home the internet didn't work, which I quickly figured out was because the DIR-655 had reset. But this meant that Wifi was on with no security, and no administrative or user passwords were in effect. Fortunately, my internal network is on another subnet (if that's the right term?) than the default 192.168.0.x that the DIR-655 resets to, and all my important devices are assigned static IPs, so they couldn't be seen by someone who might of casually or accidentally used my network via Wifi at the time.
If my network devices had been on DHCP or set to the router's default subnet, all of them would have been visible to anyone who happened to connect. This definitely shouldn't be the way a router functions and shows an inherent vulnerability; a lock down would be the better default. BTW: it's a good reason to keep the latest config backup handy... FYI!