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