Any ideas why my copy and paste are acting weird? I'm on a Macbook Pro (mid-2009) running Lion 10.7.4. Everything has been awesome these past few years, but something weird started happening a month ago: my copy/paste has been acting oddly. Sometimes I copy text and when I paste, it would be something I had copied earlier. I chalked this up to an incorrect keystroke on my part. But even when I pay attention closely this still happens somewhat infrequently.

Even more bizarre is when I cut text (just to be sure my keystrokes worked), I would see the text disappear, but when I paste what's in the clipboard, it would paste something older. huh? I'd then have to undo a few times to restore the text I cut out.

I've ruled out hardware issues with my keyboard since this happens even when I do it with my mouse (using PopClip which pops up iOS style copy/paste buttons). 

I am not running any clipboard utilities so I am stumped. If this is a hardware issue, I'd love to know that soon because my AppleCare runs out in a few weeks. Thanks for any tips or leads.
Given that this is almost certainly not a hardware issue (there is, after all, no copy/paste chip that performs that function) and given the troubleshooting steps you have included, this still sounds like a task for your local Genius Bar. If it's a configuration issue or a software conflict, they can help you sort it out, and if it's a bug in OS X, they'll make Apple aware of it so it can be fixed.

As other answers have mentioned, you still have to isolate software as a cause by trying a new user or a new install of the OS (perhaps to an external drive).