Windows Phone 7 - The messaging app icon changes to sad face when an error occurred while sending a message.
/via Wojtek Siudzinski
OS X Lion - When connecting to a Windows based PC, it shows an old CRT monitor with a blue screen as the icon.
Windows 7 - Notifications don’t fade away if you don’t touch your mouse / keyboard. If the user is away from keyboard he will see the notification when he comes back.