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Source: Microsoft Support
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Even after configuring "delay restart for scheduled installations," client machines may reboot immediately after the installation of an update.
This can occur if no user is logged into the client computer. If any user is logged in then the machine will wait for however long it is configured to wait in the group policy. If no user is logged in then it may restart immediately.
This is by design.
In the scenario where no user is logged in, this information is also logged in the windows update log (c:\windows\windowsupdate.log):
2009-03-30 11:19:27:162 1080 6a4 AU Install complete for all calls, reboot needed
2009-03-30 11:19:27:177 1080 6a4 AU Setting AU scheduled install time to 2009-04-06 10:00:00
2009-03-30 11:19:27:177 1080 6a4 AU AU setting pending client directive to 'Reboot Warning'
2009-03-30 11:19:27:177 1080 6a4 AU WARNING: Initiating reboot since no user logged on
2009-03-30 11:19:27:193 1080 6a4 AU AU invoking RebootSystem (OnRebootNow)
2009-03-30 11:19:27:208 1080 6a4 Misc WARNING: SUS Client is rebooting system.
2009-03-30 11:19:27:208 1080 6a4 AU AU invoking RebootSystem (OnRebootRetry)2009-03-30 11:19:27:208 1080 6a4 AU AU rebooting machine since no user is logged on and reboot is required.
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