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

$ go version
go version devel +c875503cf7 Sun Jun 28 03:14:10 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOOS="darwin"
GOPROXY="https://proxy.golang.org,direct"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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"

What did you do?

I implemented a buggy io.Writer which sometimes writes too many bytes (i.e., more that the length of the []byte parameter to the Write method), and used io.Copy to write to it.

https://play.golang.org/p/UeO1GCEvmNP

What did you expect to see?

A reasonable error message (indicating that the wrong number of bytes were written, or too many bytes were written)

What did you see instead?

ErrShortWrite.

Based on the documented meaning of this error variable, I believe io.Copy is using it incorrectly in this case.

ErrShortWrite means that a write accepted fewer bytes than requested but failed to return an explicit error.

For what it's worth, this came up while debugging real code: the Writer in question buffers writes and was buggy.