Wanted to share a problem I encountered on July 22, 2015. After a Microsoft Windows update, my Browser Based InfoPath forms in SharePoint that had the InfoPath Controls enabled in the IE Ribbon, failed. They would open but if you attempted any update or activity you received a Critical Error Warning. The error it showed was "Object doesn't support property or method 'getElementsByClassName' " . This was only the case in Internet Explorer. Firefox did not have this problem and the form worked from an iPad. I have other Browser based forms that do not have the InfoPath Controls enabled in the browser and they were not having this issue. If you begin to get calls that users suddenly can't use their forms, this may help. Setting is in Form Options in InfoPath Designer.
IbisDev