When you've developer unlocked your Windows Phone you can merrily deploy and debug on it and everything is great. If you've upgraded to Mango you had to run the Unlock tool again but this is fairly painless. What is not obvious is that the developer unlock has a 12 month expiry from when you first register the device and even when you subsequently use the unlock tool on the same device this never gets extended.
So one day you'll be developing as normal and get the error "Failed to connect to device as it is developer locked. For details on developer unlock, visit http://go.microsoft.com/fwlink/?LinkId=195284.". Instictively you launch the developer unlock tool, enter your App Hub credentials and unlock the phone. This completes with no errors but you'll soon find that it hasn't helped despite the fact you entered valid credentials and your App Hub account is valid.
The solution to this problem is to log into App Hub on the web and go to your account profile, select devices and remove the offending device from the list (here you'll see that the expiry date is shown). After doing this you can run the unlock tool again and successfully unlock the device.
Microsoft could certainly improve this process in the future by using a more descriptive error message and also by improving the unlock tool so that when you try to unlock a device that has expired it refreshed the registration and provide 12 more months of use (just like the manual process described above does).
Merry Christmas, and here's to a wonderful 2012 filled with Windows Phone goodness!