A few weeks ago, YouTube started showing half of their videos in HTML5 player for me. Is there a way to force it back to using Flash player only (in Chrome)?
(I'm opted out of HTML5 trial at http://www.youtube.com/html5.)
By default YouTube options will also change a bunch other aspects of the YouTube interface (e.g., hiding comments, video descriptions, ads, etc.). You may like this, but you may also want to modify some of this plugin behaviour if for example, you like video descriptions.
Try deleting your Flash cookies. http://macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html
I had the same issue, in fact I just came here because I was googling for a solution. I noticed that videos that would play in HTML5 on Chrome would play in Flash in Firefox even if I was signed in to Youtube on both browsers so I figured that it was a cookie thing. Deleting my regular Youtube cookies didn't do anything but I just cleared my Flash cookies and videos that were HTML5 are now Flash again.
After a whise wandering in forums I found this: https://chrome.google.com/webstore/detail/html5-video-for-youtube/dolajcekhnohkpncmhgledbmndjpblei This doesn't solve the problem, it's a workaround, but it works.