Apple gets a lot of credit for pioneering the use of animation in UI design. For the most part it's thoughtful and helpful. Having elements slide on and off screen gives a strong sense of physical space that helps users understand their place in an app and how the interface reacts to what they are doing.
Unfortunately the iPad has a subtle problem with one of its system-wide animations. When using the app switcher (revealed by double tapping the home button) to toggle between apps, the app you were just using moves backwards and to the left, suggesting that it is now residing to the left of the app you switched to.
This conflicts with the four-finger shortcut to swipe between apps. If you use the app switcher to switch to an app, then use a four-finger swipe to get back to the first app, the animation suggests that you should push the screen to the right to reveal the original app sitting to the left. However, the opposite is true.
Even after months of using the iPad I still get caught out by this issue. The subtle spatial cue in the app swapping animation causes me to push the screen the wrong way almost every time.
It seems an easy fix would be to change the direction of the app swapping animation triggered when using the app switching bar. I don't think this would conflict with other parts of the OS, but if anyone can think of any issues I'd be interested to hear.
Update: There may be another issue at play here as well.