I have recently aquired a small (30 user) domain structure, that was setup HORRIBLY. The previous IT Manager didn't know anything about best practices or the proper way of setting up DNS. Hell he hardly knew what DNS was!
Without going on a soapbox, I have come across an issue where various functions requiring domain access (Intranet site, Exchange, etc.) are at random times giving users a login prompt, and their accounts are not being authenticated. Even though the username and password are correct.
The only way to rectify the situation is to disable their account in AD and then re-enable it. After that it works fine.
Is this caused by something that I'm not doing correctly? Misconfiguration? Fluke?