I have a custom behavior and in it I create a hovermenu extender behavior.

Then I dispose of it correctly(the same way I dispose of all other extenders) but always get the error message at the end when I close my browser from that page that show up in a a popup window...

i create:

var yourExtendedControl = $get(RealTitlelblID);

Hi,

You can define behavior ID to each HoverMenu.

$create(AjaxControlToolkit.HoverMenuBehavior, {"PopDelay":1000,"PopupPosition":1, "id":"BehaviorID"+(i++), "popupElement":$get(BigTitlepanID)}, null, null, $get(RealTitlelblID));

But you need define different BehaivorID to each HoverMenu.

In dispose processing, you can dispose by behaiorID.

$find('BehaviorID'+(i++)).dispose();