I get an error when unloading one of my pages (reproducible for that page, but not for all pages) "undefined is null or not an object". The error occurs inside handle(), line 1903 in jquery-1.4.2.js.

Around there it says:

var events = jQuery.data(this, "events"), handlers = events[event.type];

if (events && handlers) {

...

The problem is that the events variable becomes undefined, which was supposed to be taken care of by the if statement, but it blows already when assigning handlers.