I have also had an issue with Copy/Paste reliability in Windows 8. The symptom was that unless you did a Copy operation in one window and directly topped the destination window where you performed the Paste, the Paste Operation would behave as if the Clipboard was empty. Once in a while, I believe it pasted a NUL. For instance, while using Outlook, have a new e-mail open; open IE, highlight and copy the address URL; top Outlook-InBox(main window); top the new e-mail window; paste will fail. Repeat the same, except top the new e-mail window directly and paste would work.

The problem was not Windows 8, but the "Send to Bluetooth" COM Add-in on my system that apparently shimmed itself into the Clipboard system. Disabling C:\Program Files (x86)\Intel\Bluetooth\btmoffice.dll in the COM Add-in manager made Copy and Paste robust.

Now if only I could get to "Calculator" quickly from the Desktop....