There seems to be something funky with how WebM videos get played back in IE and Edge with the native WebM components installed from http://www.webmproject.org/ie
Tested Windows 10 and Windows 8.1 64-bit.
At https://commons.wikimedia.org/wiki/Commons:Media_of_the_day pick a nice example like the ambulance video; click to pop it up and start playback, then dismiss, then do it again a time or two. Doesn't happen in other browsers or when using the ogv.js playback patch (but native WebM components will take priority over ogv.js, making the bug visible once installed).
On the second run through it'll start having multiple simultaneous playing copies of the video; only one is visible but the others can be heard.