Using go1.4.1 linux/amd64 and golang.org/x/tools 1afe783e9f23b46389ab7a439ed4e0eee558382a.

Run the following source (playground link) and then run go vet on the same file:

fmt.Printf("%[0]s\n", "arg1")
fmt.Printf("%[0]d\n", 1)

You'll see the following output:

$ go run vet.go
%!s(BADINDEX)
%!d(BADINDEX)

$ go vet vet.go
vet.go:9: arg "%[0]d\n" for printf verb %d of wrong type: string
exit status 1

There was no warning for the first format and a misleading warning for the second format.

vet does warn for negative or out of bounds indices, so for consistency I think it should also warn that indices are one-based.