We moved recently to ZK 7.0.2 and we are seeing such errors in our log:
june 09, 2014 7:00:10 AM org.zkoss.zk.ui.impl.DesktopImpl service
SEVERE: [Desktop z_y9w:/hindamine/index.zul] client error: Failed to process
setAttr
Cannot convert 'j.$n("hdfaker-bar")' to object (TypeError)

It seems that users using Opera 9.80 (MacOS and Windows 7) are getting this error.