Looks like someone uploaded an HTML5 video or audio file with the wrong encoding. It can only validate the extension and not the encoding of the media so this can happen. Could also be the below browser bug if you're using FireFox (it's already fixed; be sure to update your browser).
bugzilla.mozilla.org/show_bug.cgi?id=1370175
Also the picture popups from the activity and other areas are not displaying at all at times, and when they do display it is incorrect
You'll need to be more specific regarding this. All my tests are showing this working perfectly fine; including tests on our demo site.