Cmd-Shift-Tab behaves differently when on dual screen On Mavericks, Cmd-Tab works as expected just fine (selecting the next application in the MRU list). With the task selector open, I can hit shift as well to move back in the same list before selecting app.

As soon as I plug in a second monitor (with it's own "space" I assume), this suddenly works differently. When I activate the task selector with Cmd-Tab, it opens as usual, but as soon as I hit shift in addition, the task selector closes, selecting whatever was selected when I hit shift.

On dual screen I can start the task selector up with either Cmd-Tab or Cmd-Shift-Tab, but as soon as I add or remove shift from it, it selects the application underneath and closes the task selector. On a single screen, I can move back and forth without anything getting selected until I release Cmd-Tab as well.

Does anybody know if this is a bug or the way it is supposed to be, and if the latter, what is the rationale for this?
samh nailed it in the comments. The issue is the use of an external bluetooth keyboard, and has nothing to do with dual monitors / spaces. The dual monitors are part of my office setup, which also has a bluetooth keyboard. I noticed that when using the computer at home (away from the office), the problem went away, and for some reason concluded it had something to do with the dual screen setup (forgot all about the keyboard).

The keyboard is a Logitech Bluetooth Easy-Switch Keyboard K811. I will see if I can find any keyboard setup and/or driver updates that fixes it. Very annoying.

Update: Updated to latest K811 drivers, no change. Googled a bit more and found:

http://forums.logitech.com/t5/Keyboards-and-Keyboard-Mice/K811-left-shift-key-quot-flickers-quot-when-used-together-with/td-p/1066141

tldr; Logitech is probably aware of it, but do not care enough to fix it. It's a great keyboard and Logitech should fix it, but it's hard to recommend it until they do. Switching apps is kind of a core function in a window system...