I did already experienced with Netscape, and now with Galeon,
that browser freezes together with all desktop if something
happens while you are scrolling some pull down menu like bookmarks.

Here how to achive this freeze consistently:

Try to load URL which does not exist. While browser searches for it,
pull down one of long menu (bookmarks). If the dialog that URL is not found
apears while you are scrolling the menu, you get the freeze.
Dialog with OK button to be pressed appears below
the pull down menu, is unaccessible, and control is gone.