I found a semi-serious bug in DIR-655. I have hardware revision B1 and noticed this on 2.07NA and 2.10NA. It may exist elsewhere. It is random and does not always happen, making diagnosis very difficult.
While trying to get my FTP server working (see
http://forums.dlink.com/index.php?topic=51194.0) I noticed this problem. I was making changes to virtual servers. However when rebooting to make the changes take effect the router would no longer allow Internet through and was broadcasting an insecure wifi network with the SSID I had set. Checking those settings they were all fine (it thought it was still doing WPA2 personal, even though it was not). So I disabled the settings I had just changed. The problem persisted! Even editing the wifi settings and saving, restarting, and power cycling had no effect. Still no Internet and no wifi protection.
The only fix was to reset the settings to factory default and things would be okay again. But randomly when changing settings (only noticed this with virtual server, port forwarding, and DMZ) it would occur again. I updated the firmware to 2.10NA and problems would still randomly show up. Sometimes restoring a configuration from before these problems showed up would cause the problems to show up as well.
I know that this probably cannot be fixed without knowing exactly what was happening, but being a random occurrence I cannot give any more information.