I was playing with HTML5 <video> tag recently. Everything was working fine with Safari and Chrome; however IE9 constantly stopped playing the video at ~11-20 seconds. I thought the problem was in compatibility, but, after spending half of the day (and night), I figured out the problem.
The problem was the "Sophos Web Content Scanner" add-on (Sophos v9.5). Once disabled (from the menu, Tools -> Manage add-ons), <video> tag works as it should with IE9.
If you have Sophos installed, you can test this by using, for example, this link videojs.com (or HTML5 version of YouTube). IE9 needs to be restarted each time the add-on is enabled/disabled.