The video is a locally uploaded video, and is a WebM video. Transcodes are disabled. I recently tested using an ogg video and did not encounter the problem.

The following comes from a Firebug debug console:

I am using MediaWiki 1.25.2. The TimedMediaHandler version is REL1_25: rev c4274e4. Firefox 40.0.2 and Chrome 44.0.2403.155. On Chrome there is a similar problem except it does not have the notice. The OS is Windows 10. I have not yet made a public testcase. I also tested using Firefox 40.0 on Ubuntu 14.04 and same problem.