In the example app, it is possible for the scroll view to stop in the wrong place. You can reproduce this by swiping rapidly with multiple fingers, sometimes in opposite directions. After it gets to this state, swiping works as normal, until it happens again.

I looked at the code to see if there was anything obvious going wrong but I didn't find anything.
This happens on an iPhone 6 running iOS 8.4 and the Simulator running 8.4 and 8.1.