Copy & Paste doesn't work
From this Apple Support discussion:


  So, to fix a stuck clipboard (pasteboard) problem, you simply need to:
  
  
  Open your application folder
  Open the utilities subfolder
  Open the Activity Monitor app
  Type “pboard” into the search box at the top right
  In the search results below, there should only be one result — a row listing the “pboard” process. Highlight it by clicking on it.
  Click the “x” button at the top left of the window which represents ‘quit this process’
  
  
  Once you’ve done that, OS X will kill the faulty PBoard process and
  re-establish it with a fresh one automatically. Your clipboard should
  be back to normal.