UPDATE: AHS has been updated to v0.3.9.7 to eliminate the YUI Event.addListener(window, 'resize', function) issue!
Case in point, I recently had to track down a mysterious bug in a YUI-based script I had written. The bug popped up in Firefox, to my great surprise, and I spent hours tracking recent changes trying to find the cause. After discovering that the bug didn't occur in Firefox on other machines, I began to disable Firefox extensions. It turned out that the autoHideStatusbar (AHS) extension that I had recently installed conflicts with onresize YAHOO.util.Event.addListener instances in YUI.