mz 4.1.3 also showed the error (aw is going to update it). I'm inclined to think the bug's still in all those versions of mzscheme, it just takes forever to express in certain situations that we don't yet understand.
ubuntu+v372 hasn't shown the bug after several hours. I'm going to try retreating to arc3.1+mz372 for readwarp.
If v372 is good, PG's comment dates back to arc2, so whatever he may have seen seems to be unrelated.
