Okay lets start by explaining this bug. What I did was I did a bit of bugfinding (which I do occasionally) and found this.
The 'when this sprite clicked block has a delay with it. It pauses after it is clicked which can be very annoying.

Please note that this is only a bug in the project editor, not in the viewer.

This example project http://scratch.mit.edu/projects/11428152/#player Shows what I mean. In the project viewer it works fine but if you go into the project editor and click quickly the background won't change showing the delay.

Well, when you click in the editor, it redirects you do the thing you click.
So if you were working on the stage's scripts, and decided you wanted to work on Scratch Cat scripts, you can click on the scratch cat to do that.

Not for the background

Open up a generic 1 sprite cat project, go to the Scratch Cat's scripts, and then click on the stage.

dracae wrote:

Open up a generic 1 sprite cat project, go to the Scratch Cat's scripts, and then click on the stage.

Not to sound rude or anything, but that isn't the problem as I can prove that it isn't.
What I did was import the scratch cat into the project. If you click quickly once or twice on the background it won't select the background but the background won't change. I have edited the project to show this and prove my point.

Ok, I think I have an idea.

The stage doesn't respond if you use a <touching mouse pointer?>
You need to be touching the mouse point if you want to be clicked.
Thus, this glitch maybe?