Setup a frozen grid with a footer like here:

http://zkfiddle.org/sample/14eq7fl/1-Grouped-Frozen-Grid-Footer-Bug

Open the browsers console, then begin scrolling the frozen area. On scrolling the console will get filled with the following bugs:

Uncaught TypeError: Cannot read property 'style' of undefined zul.mesh.wpd:3333
zul.mesh.Frozen.zk.$extends._doScrollNow zul.mesh.wpd:3333
zul.mesh.Frozen.zk.$extends._doScroll zul.mesh.wpd:3260
(anonymous function) zk.wpd:13822
jQuery.event.handle zk.wpd:2985
elemData.handle.eventHandle zk.wpd:2619

Also the width of the columns is NOT calculated correctly. This might not look so bad for this example, but if you have setup more columns in the frozen area, things get even worse.