I'm not sure if this is what have happened to all, but here is what happened to me, and how I "solve" it:
I have a javascript on <body onload... that displays a "Waiting" animated GIF and a message that says "Pleas wait".
When the page has finished loading, the message and the GIF dissapears.
Now,,,,,, with the CollapsiblePanelExtender,,, something weird has happened.
I have a page with the CollapsiblePanelExtender, the "Loading" javascript and a GridView with EditItemTemplate in it.
Sometimes (not always), the page has finished loading, but the "Loading" message remains in the page,,,and if I collapse the panel, it goes away and everything works fine so on.
My temporal solution was simple and in my current scenario works for my purposes,,,,, Since everything works fine after I collapse the panel, what I did was to add this line of code at the end of the Page_Load
Me
.cpeHeader.Collapsed = True
(where cpeHeader = CollapsiblePanelExtender)
Since I don't have a problem having the panel collapsed when the page loads, I can live with this.............