Emacs version: 24.3.1
Platform: Windows 7, confirmed in Linux

Problem:

Windows users who set up Emacs to cut/paste from the kill-ring get empty
paste results.

Details:

Windows users who are having difficulty with cut/paste behavior are often
advised to do the following:

(setq select-active-regions nil)
(setq mouse-drag-copy-region t)
(global-set-key [mouse-2] 'mouse-yank-at-click)

But if, in the process of pasting text, you click, then drag the mouse even
a few pixels, this causes an empty string to go into the kill-ring, so a
subsequent paste inserts nothing, but an examination of the kill-ring
indicates the desired paste value has been pushed down by the empty string.

Steps to reproduce:

1. Enter lisp-interaction mode.

2. Set up the "normal behavior" by executing this form:

(progn
  (setq select-active-regions t)
  (setq mouse-drag-copy-region nil)
  (global-set-key [mouse-2] 'mouse-yank-primary))

3. Select some text buy dragging mouse-1 over it.

4. Move to a different area in the buffer and set point with down-mouse-1,
but move the mouse a few pixels so you see the "Mark Set" message.

5. Paste the text with down-mouse-2.

6. Observe the correct behavior (text gets pasted).

7. Now, set up the "modified behavior" by executing this form:

(progn
  (setq select-active-regions nil)
  (setq mouse-drag-copy-region t)
  (global-set-key [mouse-2] 'mouse-yank-at-click))

8. Repeat steps 3 thru 5.

9. Observe that nothing is pasted.

10. Execute the following form to "fix" the problem:

(progn
  (setq kill-ring (remove "" kill-ring))
  (setq kill-ring-yank-pointer kill-ring))

11. Do the mouse-2 paste again, observe that it worked this time.