Just ran into a very unexpected issue. I've been developing an app for the past two months, mostly AJAX work. Turn off debugging for the first time today to do a production deploy and boom, I get errors. Long story short, turns out that in debug mode, a missing initializeBase call doesn't really matter. ASP.Net Ajax must be doing the extra plumbing for me. In release mode though, the method call is required before the prototypes are flushed out.

So, if you do a release build and get the nice error that "a.beginUpdate is undefined', check your intializeBase calls.