What version of Go are you using (go version)?

go version devel +36323e4 Tue Nov 7 01:50:16 2017 +0000 linux/amd64

Does this issue reproduce with the latest release?

In go vet 1.9, yes, but now it is an issue with the new go test on tip.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOOS="linux"
...

What did you do?

custom_test.go:

package custom

import (
    "fmt"
    "testing"
)

type custom int

func (custom) Format(fmt.State, rune) {}

func Test(t *testing.T) {
    fmt.Printf("%z", custom(0))
    fmt.Printf("%#z", custom(0))
    fmt.Printf("%#s", custom(0)) // unrecognized flag
}

What did you expect to see?

$ go test
PASS

What did you see instead?

$ go test
# custom
./custom_test.go:15: Printf format %#s has unrecognized flag #

Some code in okPrintfArg, in cmd/vet/print.go, handles custom formatters already. This case might be handled similarly to the way it passes unrecognized verbs for custom formatters.