Mountain Lion spaces erratic behavior

I have been noticed unique behavior between virtual desktops on my Macbook. For example, Mail is in Space 1, my browser in Space 2, and Autocad in Space 3. Seemingly at random intervals, it will change Space 2 and Space 3, so I end up with, basically Space 1, 3, 2 (but they are labeled space 1, 2, 3).

Is this a feature, or something that can be fixed?

I'm not sure if it is related to the question, but I have also noticed that sometimes when I do the four finger swipe to change spaces, it jumps two spaces.

You can uncheck "Automatically rearrange spaces based on most recent use" on the "Mission Control" tab in system preferences. This will leave the desktops in the expected order.

My observation is that the "spaces" (now called "desktops") in Mountain Lion are sorted in a "most recently used" type order. That is, the more often a switch to a particular desktop, the closer to the "front" of the desktop list it is moved. If you enable the 4-finger "swipe up" Mission Control gesture, you can see the desktops in their "swipe" order.

I suspect one reason for this is that "full screen" apps get their own desktop, so the number of desktops varies dynamically, and the simple number/grid scheme from earlier OS X versions no longer can apply.

I tend to switch apps with command-tab, so the absolute placement of desktops is not so relevant to me. I do occasionally miss the ability to switch to a particular desktop with control-number, but have gotten used to using "swipe up" to find and select the desktop I want.

If you want to associate an app with a desktop of its own, switch to the "last" (empty) desktop, start the app, and select Options->Assign to This Desktop from the context menu in the dock for that app. If the app supports full screen mode, simply switch it to full screen and it will create its own desktop.

I was going to include a sample "Mission Control" screenshot below, but I haven't answered enough questions here yet, so I've posted it on dropbox instead. You can use the 4-finger left/right swipe to switch desktops in this view, or you can click on the thumbnail of the desktop you want to switch to. Across the top of the screen are the desktop thumbnails. Spotify is in full-screen mode. Desktop 4 has no apps on it. If I put an app there, I will get a new empty Desktop 5. I'm currently on Desktop 1, which is shown "exploded" below the thumbnails.