This started happening a couple of days ago, and happens with two different images. Haven't done any system updates for a week or more, nor added any actions or plugins. I do have a Wacom tablet that occasionally runs amok* until I unplug and replug its USB cable. It did that recently, but can't remember when. I can clip two layers by using opt+click on the little icon that loads an adjustment layer. Running PSCS6 v 13.0 64 bit on an iMac with OSX 10.7.4, 650 GB disk storage (639 GB capacity, 134 GB available), 4 GB RAM. I've probably done something to make this happen, but sure can't figure out what.
*By running amok I mean that it randomly changes focus between everything I have installed, including apps that aren't currently active (I think . . .)
In order to be certain,do you press and hold down the Opt key before you move the cursor between the layers? Try the trackpad or mouse on your system instead of the Wacom
If the problem is still there, reset the workspace by going to the far upper right corner of PS and in the dropdown menu under the workspace name,"Essentials" as an example,chose "reset workspace."
Last thought would be to reset Photoshop Preferences if any of the above does not work.
Macintosh - CMD-SHIFT-OPTION immediately after you double-click the PS icon.