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

go version go1.10 linux/amd64; exists since 2010

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/twmb/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/twmb/go"
GORACE=""
GOROOT="/home/twmb/go/go"
GOTMPDIR=""
GOTOOLDIR="/home/twmb/go/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build085502795=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/BtWWmSrhxZk

What did you expect to see?

err: lim 1, wrote: 0
err: lim 2, wrote: 0
err: lim 3, wrote: 0
err: lim 4, wrote: 0
err: lim 5, wrote: 0
err: lim 6, wrote: 0
err: lim 7, wrote: 0
err: lim 8, wrote: 0
err: lim 9, wrote: 0
ok: lim 10, wrote: 1

(continuing with one character in header)

err: lim 10, wrote: 0
err: lim 11, wrote: 0
ok: lim 12, wrote: 1

What did you see instead?

err: lim 1, wrote: 1
err: lim 2, wrote: 2
err: lim 3, wrote: 3
err: lim 4, wrote: 4
err: lim 5, wrote: 5
err: lim 6, wrote: 6
err: lim 7, wrote: 7
err: lim 8, wrote: 8
err: lim 9, wrote: 9
ok: lim 10, wrote: 1

(continuing with one character in header)

err: lim 10, wrote: 10
err: lim 11, wrote: 10
ok: lim 12, wrote: 1

This is due to