Go spec says:

Converting a signed or unsigned integer value to a string type yields a string containing the UTF-8 representation of the integer. Values outside the range of valid Unicode code points are converted to "\uFFFD".

1. cmd/compile rejects string(1<<100) instead of yielding "\uFFFD": http://play.golang.org/p/1s6ge-XFqd (gccgo also rejects this; gotype accepts it.)
2. runtime incorrectly truncates 64-bit integer values to uint32 before checking if they're a valid Unicode code point: http://play.golang.org/p/M_Gkll5VY-