We've recently encountered an interesting problem:
One of our applications had a service using .net remoting with impersonation turned on.
Inside this service we had some code using WindowsIdentity.GetCurrent() to find the user in AD and check some of its properties.
Our issue was thus:

  1. User X logs in and uses the service. All is well.
  2. User X has its LogonName and SAMAccountName changed to Y
  3. User Y logs on and uses the service.
  4. When the service is calling WindowsIdentity.GetCurrent(), the username returned is X, even though the name has changed.
  5. After restarting the server (not only the service, the entire OS), All is well (until the next time a user chnages the LogonName).

We eventually discovered that WindowsIdentity uses LsaLookupSids, which caches the SID/username mappings instead of asking the DC again. To prevent that, there is a registry value that can be changed to stop caching, according to kb946358:

System\CurrentControlSet\Control\Lsa:LsaLookupCacheMaxSize=0

I made it into a little script, that we used as a mid-term solution to clear the server's cache without disrupting service:

param([string[]]$servers)
$servers | %{
 $base = [microsoft.win32.registrykey]::OpenRemoteBaseKey('LocalMachine',$_)
 $sub = $base.OpenSubKey('System\CurrentControlSet\Control\Lsa',$true)
 $sub.SetValue('LsaLookupCacheMaxSize',0)
 Start-Sleep -Seconds 5
 $sub.DeleteValue('LsaLookupCacheMaxSize')
 $sub.Close()
 $base.Close()
}

Enjoy!