Consider this, “GIFV good”: http://i.imgur.com/SQXB06O.gifv
Consider this, “GIFV bad”: http://i.imgur.com/MHfD9WX.gifv
Now both of these are recent uploads; 13 and 15 hours old respectively. However one plays as a video and one plays as a GIF. You can tell because if you right click GIFV good it says Pause, while if you right click GIFV bad it says View Image.
I know that GIFV bad has a video available:
$ wget --spider http://i.imgur.com/MHfD9WX.webm HTTP request sent, awaiting response... 200 OK Length: 542519 (530K) [video/webm]
However if you try http://i.imgur.com/MHfD9WX.webm directly, it just redirects you to the GIFV and plays a GIF.
What is causing this behavior?