Reproducing sample:

 
<zk>
	<popup id="help" onOpen='help.appendChild(new Label("dynamic help text"));' />
	<div id="container">
		<button tooltip="help, after_start" label="click to replace" 
			onClick='container.getChildren().clear(); container.appendChild(new Label("replaced button"));' >
		</button>
	</div>
</zk>

The button has a tooltip, with positioning after_start.
Clicking the button will replace the content of its surrounding container (usecase e.g. like clicking a button in a listbox -> switches to a details view)

When hovering the button, the tooltip with dynamic content will open after a short delay and after a server roundtrip - OK.
when clicking the button at the "wrong time" (while the tooltip is loading), a client error is caused (reproduced in chrome and IE so far)

(I could not reproduce without "after_start", I think the positioning is important, because the mouse event will be received by the tooltip instead of the button, if no specific position is used).

Uncaught TypeError: Cannot read property 'style' of undefined zk.wpd:10978
zjq.dimension zk.wpd:10978
zul.wgt.Popup.zk.$extends._posInfo zk.wpd:23538
zul.wgt.Popup.zk.$extends.open zk.wpd:23459
zk.override.open zk.wpd:25225
_tt_open_ zk.wpd:20297
(anonymous function)

It might take several attempts to reproduce, see the following screencast http://screencast.com/t/eeII2ltg3