To reproduce on latest emacs-24 branch:

  mkdir /tmp/foo.d
  emacs -Q /tmp

Dired mode creates links out of the directory names, such as foo.d.
C-h k then mouse-1 (ie left) click on a link. See:

  <mouse-2> (translated from <down-mouse-2> <mouse-2>) at that spot runs
  the command mouse-yank-primary, which is an interactive compiled Lisp

But if you C-h k then mouse-1 click where there is no link, see:

  <down-mouse-1> (translated from <down-mouse-1> <down-mouse-1>) at that
  spot runs the command mouse-drag-region, which is an interactive

Though this doesn't create an issue in Dired, it is causing another
Emacs package I use (unreleased) to paste text instead of doing what
it used to do in Emacs 24.3, which is to show its own context menu.

Why would C-h k mouse-1 say anything about mouse-2?

In GNU Emacs 24.3.92.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10)
 of 2014-06-27 on REDACTED
Windowing system distributor `The X.Org Foundation', version 11.0.11405000
System Description:    Ubuntu 12.04.4 LTS