I've been using Chromium as my browser for a while and everything has been great until recently, when some youtube videos wouldn't play reliably and instead would show the error:
This is apparently caused by being enrolled in Youtube's HTML5 video trial. You can enroll/unenroll from the program by visiting this link.This video is currently unavailable.Learn more
I had forgotten I was even using HTML5 video, so I never even considered this could be the problem. Once I unenrolled, everything works fine again. Of course, YMMV.