The Problem: This error message ("YAHOO.util.CustomEvent is not a constructor") appears after I update my YUI library from 0.12 to the latest release (2.3). There was no such error before the update.
The Why: The new Container in YUI implementation fires CutomEvent at various stage, thus is dependent on the event utility. In my old web pages, the .js files happened to be included in the wrong orders (container.js was included before event.js) and caused this error message.
The Fix: include "event.js" before "container.js". Better yet, try the new YUILoader utility which claims to be able to load the correct libraries automatically.
The Lesson: Whenever you see an error like "[XYZ] is not a contructor", try to check for typo then file dependence.
Link: Also see this thread on YUI discussion group: