Something has been bugging me with OS X for a while — sometimes the OS X clipboard (officially known as "pasteboard") gets stuck and won't accept any new 'copied' content. Instead, when you 'paste' in any app, the clipboard always pastes back the last thing you successfully copied.
One solution to this is to reboot the Mac, but since Mac OS X is generally so stable and doesn't need to be periodically rebooted as a matter of course, rebooting a machine with many open apps and windows can be a hassle.
Searching for this issue online provides a lot of bad advice (one website I saw said "repair permissions and if that doesn't work, reinstall the OS"… terrible advice.)
Other websites incorrectly advise to kill the PBS process and restart it.
Apple's manual page for PBS notes it is not related to the Pasteboard process. Instead, the correct process to kill is PBoard.
So, to fix a stuck clipboard (pasteboard) problem, you simply need to:
1. Open your application folder
2. Open the utilities subfolder
3. Open the Activity Monitor app
4. Type "pboard" into the search box at the top right
5. In the search results below, there should only be one result — a row listing the "pboard" process. Highlight it by clicking on it.
6. 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.
I have a suspicion that this stuck clipboard is a bug in OS X Mavericks 10.9 – I don't recall it happening before I installed that. It is still an (infrequent) current problem as at 10.9.2.