On a touchscreen, a single finger scroll starts registering as a multi-finger zoom gesture after certain interactions with webpage elements. Scroll up starts registering as zoom-out and scroll-down as zoom-in. Single finger events including clicks stop registering after this has happened, even after reloading the page.  
  
I can consistently replicate this on full-screen HTML5 videos (eg on youtube) after touching the "seek" bar at the bottom of the video. Also on any duck-duck-go search page (example: https://duckduckgo.com/?q=webkit), after tapping the "settings" buton on the top right of the page.  
Note: Clicking these elements with the mouse does not produce the bug, only when tapping them with the touchscreen.  
   
I wonder if the complex html5 elements on these pages is causing this (the settings button on duck-duck-go is supposed to animate a slide-out pane for example).