I copied something to the clipboard, but as of lately, neither Ctrl + V nor Edit -> Paste will actually paste it into the Google Docs document. Does anyone know what's happening? I'm using Vista/ Firefox.

Dragging & dropping a text bit from one window into the Google Docs doc does work, though.

Perhaps limitations due to (unvisible) HTML copied
=> paste first what you have copied into a textbox (or Notepad)
=> then copy what you just pasted
=> you can now safely paste your original text.

I have been having the same problem for a week or so now. It's not an issue of formatting of the copied text, as I've tried the plain text solution TOMHTML suggested. Hopefully Google fixes this.