Sometimes we see lines containing
<brdata-attributes="" /> and <brdata-mce-bogus="" />
appearing in our texts. I am not sure why and when this happens but after looking at the code it was pretty obvious that a small bug in
tinymce/plugins/mw_wikicode/plugin.js was causing this.
Line 1701 reads
return '<br' + decodeURI(attributes) + ' />';
this has to be
return '<br ' + decodeURI(attributes) + ' />';
imo (note the added " " after "br").