Unicode breaks in the hello-world webapp. E.g. if you write
(defop hello req (pr "hello world \u1234"))
You get some strange looking text in you browsers. This seem to be because arc is generating UTF-8 output (which I think is MzScheme default) but not declaring the encoding, which will make most browsers default to interpret it as iso8859-1.
It seem to be fixed by changing svr.asc line 105 to