There has been a number of error reports recently (first report was 2015-11-12) about MediaViewer showing the error page when trying to view an image. The errors are not reproducible, but seem to happen frequently for the affected users.
Based on developer toolbar screenshots, there are two different types of error:
* failure to load the thumbnail (this is probably for most of the errors). Chrome does not show any useful error message, just `net::ERR_SPDY_PROTOCOL_ERROR` and aborts the request (says "Provisional headers are shown" in the inspector). Possibly some kind of CORS error.
* requesting a thumbnail with the exact size of the image (cf. T75935). E.g. https://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Buell_1125R.JPG/461px-Buell_1125R.JPG when the correct URL would be https://upload.wikimedia.org/wikipedia/commons/e/ef/Buell_1125R.JPG. This is a bug in MediaViewer's [[ https://github.com/wikimedia/mediawiki-extensions-MultimediaViewer/blob/master/resources/mmv/provider/mmv.provider.GuessedThumbnailInfo.js#L91 | thumbnail URL guessing ]] logic (patch attached). The impact is minimal - it only affects small images with uppercase file extensions, and even then there is a fallback mechanism (which doesn't work due to the other error).