Strange things happening with LR started with V5. I uninstalled and re-installed, and now I'm on CC, still having probs.
Issues are intermittent, but fairly regular. The only thing I can see that is consistent is that the problems seem to arise when the prog is reported as using a high memory amount (over 1.5Gb - I have 8 installed) as reported in Windows Task Manager.
Todays issue is that I have three images selected, I hit x to mark as rejected, and the 3 images PLUS the 3 previously selected images now show an X. Hitting U removes the X from all 6 images. Hitting X again marks all 6 as rejected - and so on.
I had the same problem trying to add a color label.