with the old toolbar button are inserted through a fragment of javascript looking like
<script>if(window.mw){
mw.loader.using("mediawiki.action.edit", function() {mw.toolbar.addButton(...
...
$(function() { mw.toolbar.init(); } );
});
}</script>
init fail in some case because the selector in /resources/mediawiki.action/mediawiki.action.edit.js : $toolbar : $( '#toolbar' ), doesn't return the toolbar div element, when failure occur document.readyState == false, ditto for $.isReady, look like mw.toolbar can be init before the DOM is fully built.
This is not easy to reproduce, for some user it fails always, for other never and so on.
Version: 1.19
Severity: major