This one didn’t actually involve any code changes. Rather, the Acid3 test itself was changed as a result of feedback from the browser developers. The issue was some rather obscure details of click() DOM method which are not specified anywhere but are implemented in certain way by all browsers (more details in this thread). The behavior expected by earlier versions of the Acid3 was different and would have broken some web content. The ongoing HTML5 work will allow standardizing these kinds of details.
It is good to see that besides testing many exciting new standards Acid3 also takes real-world web compatibility seriously.