Some applications (including base ones, like the Clock application, or the Media Player application) save power by not refreshing the display when they're not the topmost active application on the system. This works well enough right until you hit the task switcher key, and you see outdated thumbnails in the task switcher grid.

For example, if you open the Clock application and let it run in the background for a while, after you hit the task switcher button you'll see that the clock is outdated in the preview. This is documented in bugs.maemo.org bug #5466.

If you open the Media Player application, you'll see that the time slider doesn't refresh when it the background. Thus, you cannot really guess the current song position by looking at the media player thumbnail in the task switcher grid; you have to actively switch focus to the media player and let it refresh.

Of course, this is far from ideal, even from an user experience point of view. But we also want to save power...