WP Shopping Cart by Instinct Entertainment causes the drag ‘n’ drop of widgets to stop working. I tried activating & deactivating it several times and it happened every time.
Broken Link Checker by Janis Elsts cannot be deleted from the Plugins page (after you click delete you get a blank browser window, regardless of whether you are doing a bulk or individual delete).
Just FYI. Both of these results were obtained when using Firefox 3.0.11