Please answer these questions before submitting your issue. Thanks!

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

go version devel +9e8fa1e Fri Jun 17 15:18:39 2016 +0000 darwin/amd64

1. What did you do?

Make a http.Client with a Timeout. Cancel the request.

https://play.golang.org/p/v-QbAxdj-3

1. What did you expect to see?

If the cause was not a Client.Timeout, then the error should be left alone.

net/http: request canceled

1. What did you see instead?

The error is

net/http: request canceled (Client.Timeout exceeded while reading body)

This is because reqWasCanceled returns true whatever the reason for the cancelation is, be it a library user cancel or a Timeout. Then cancelTimerBody.Read uses that function to add the message.

The (Client.Timeout exceeded while awaiting headers) message is more likely to be correct (even if slightly racy), because it only adds the message if !deadline.IsZero() && !time.Now().Before(deadline).