This is curious. I've ran it with out-of-the-box arc3.1 and out-of-the-box mzscheme 4.2.4 and 4.2.2, on the same computer (with Mac OS X 10.6.2). It's erred every time with 4.2.4 (I think two or three times very quickly and three or four times after a long time), and it has not erred once with 4.2.2 (after four or five tries). I suppose it must be a problem with mzscheme.
