Panning beyond the peek or reveal amounts causes unwanted UI issues.

When the under left or under right view controllers are showing and the top view controller is panned further off the screen it reveals the edge of the under view controller. The Facebook app prevents further panning past this point.
![iOS Simulator Screen shot Dec 15 2012 3 05 02 PM](https://f.cloud.github.com/assets/963659/15087/6cd5545a-46f3-11e2-88c0-53678977e371.png)