Windows NT 3.51 unintended shutdown
After logging in and out of the console of a Windows NT 3.51 fileserver many times, it had become routine. Click and hold File in Program Manager, release the mouse button on Logoff (the menu item right above Shutdown) and click OK on the “This will end your Windows NT Session” dialogue box that followed. Until the inevitable happened: I released the mouse button a little late. And automatically clicked Ok. And watched the server shut itself down.
I went to the phone, waited for the calls and explained that some emergency maintenance was underway, but it would not last more than 10 minutes. Within 10 minutes, the server was back up. One of the few times IT correctly predicted how long maintenance would take.