A few days ago I opened a youtube video in chrome incognito mode so it won't pollute the feed, but in the middle of the video I decided it's quite good so I opened it in the main window. At this point the tracking code glitched out and the video started where I left it off in the incognito mode. Couldn't reproduce it since though.
Incognito mode isn't as incognito as normal users might think.<p>If you have at least a little technology knowledge it should be clear what that mode is.