See http://lists.w3.org/Archives/Public/www-dom/2012JanMar/0007.html and http://lists.w3.org/Archives/Public/www-dom/2012JanMar/0008.html
Thanks for the report!
Anne, you didn't change the return values of getElementsBy* on Element but that they should also be HTMLCollection, right?
Oops, my bad. They DO return HTMLCollection. I must have been reading some stale version of the spec :( Sorry about the confusion.