Sometimes, MediaViewer just doesn't work - you click on the image and end up on the file description page. This happens randomly, and I have observed it sporadically for some months now. It seems to happen when I visit a wiki that I haven't visited lately (so I suspect it is somehow related to caching). The problem persists after page reload, but it goes away eventually (no idea exactly when), and after that everything works fine (including the exact same images). When the problem appears, mmv.bootstrap is loaded but the wgMediaViewerOnClick config variable is false for no apparant reason. There are no JS errors.