This one doesn't actually look like a bug to me. Although the read error is not returned on the initial call to decoder.Read it is saved in d.readErr and neither lost or overwritten. It will be returned on the next call to decoder.Read. This behaviour is compatible with the io.Reader specification.

When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call.

If we insert a second call to decoder.Read directly after the first, e.g.,

  n, err := decoder.Read(buf)
+ n, err = decoder.Read(buf)
  if err != fakeErr {

the test passes as fakeError is returned.

Ah, you're right! Here's an updated test: https://play.golang.org/p/J5KWJI-iHW