Formatting any slice (or array) but []byte in binary using the %b format code works. For example:

fmt.Printf("%d", []int{1,2,3}) -> [ 1 10 11 ]

but if the members of the array or slice are byte then the special case fmtBytes() is invoked, and it lacks a %b case. So instead the formatting fails:

fmt.Printf("%b", []byte{1,2,3}) -> %!b([]uint8=[1 2 3])

Looking at the code in src/fmt/ I don't think it would be hard to not only make %b work on []byte, but to also make it work nicely, like %x and % x do, giving something like:

fmt.Printf("% b", []byte{1,2,3}) -> 00000001 00000010 00000011

which is the binary equivalent of the special case % x operation on a []byte, which, for reference, produces:

fmt.Printf("% x", []byte{1,2,3}) -> 01 02 03

I see this behavior in go 1.5.1 on linux/amd64 and linux/arm, but the behavior isn't new, nor is it specific to those OS/ARCH combinations.