Well, good questions. Firefox on Mac (v.7), unfortunately my default browser, is the only one that seems to fail outright - sometimes Firebug will let me click on the error to continue, sometimes the page just freezes.
All other browsers I've tested (Safari Mac, IE 9 on Win7, Firefox on Win7) spew the same sorts of errors, and lots of them (xxxxx is undefined, also dojo.io.script error") while zooming, but the browser doesn't seem to care.
I unminified the 2.4 dojo/esri js file, and verified that it is the base map tiles (cache?) where the undefineds are popping out from.
I do not at this point have a small "test case" which exhibits this problem, only the large app.
Rex Bradford