I've had this problem for a little while now (can't recall what would have triggered it). I'm using Safari 6.0.5 on 10.8.4 on an early 2011 15" MBP.
When I try to copy anything using the keyboard from google docs nothing happens - I also hear an audible error noise. I think it's Safari saying "HAHA - sucker". Anyhow, I can paste into GDocs with whatever is in the clipboard at the time. I also use PopClip, which gives you the option to copy text when something is selected - that works as normal. If I try to copy something from the edit menu, I get the "These actions are unavailable via the Edit menu, but you can still use:" error dialog.
Cleared cash, disabled plugins, restarted computer...etc...none of the typical troubleshooting stuff works.