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HTTPS or Subversion.
OS X Mountain Lion 10.8.2
Safari 6.0.2 (8536.26.17)
I can reproduce this very consistently (though it may take two or three attempts) across all of my machines running the above configuration. To reproduce:
1) Navigate to http://www.reddit.com
2) Click any link
3) After a few seconds, navigate back to Reddit using a two-finger swipe (not the back button)
After the above steps, I am unable to scroll for a second or two, and then the page disappears altogether.
This issue does not occur if I press the back button.
You can see a video of this issue here: https://www.dropbox.com/s/0xy4vniad0yxg39/Bug.mov
I can reproduce this, but only in Safari. I don't see the problem in Chrome or Firefox. Possibly due to the swiping transition animation?
I'm running the same Mountain Lion/Safari versions.
I've not seen it on any other site, as far as I'm aware. Not to say it isn't something Webkit-related, but I've only ever seen it reproduce on Reddit.