io ReadAtLeast doc
// It returns the number of bytes copied and an error if fewer bytes were read.

That part doesn't say anything about the error returned if enough bytes are read, but later in the comment:

On return, n >= min if and only if err == nil.

I think this could be clarified to:

On return, n >= min if and only if err == nil. In particular, if r.Read() returns an error but fills up buf, that error will get dropped.

The persistence of an error in a io.Reader is not specified in the documentation and it is reasonable that there is a class of errors that are transient and do not cause an issue if Read were called again. Furthermore, there's nothing that you can do in ReadAtLeast since it is a stateless function and cannot affect the stickiness of an error on the io.Reader passed in.

This is only relevant to documentation; documentation fixes are okay for Go1.9, but leaving in Go1.10 milestone.

You're right about persistence of errors in io.Reader, but that's not what I'm referring to here. The loop in ReadAtLeast stops as soon as r returns an error, but as long as enough bytes are read, that error is dropped. That's the behavior that I'm making a case for being documented.

Change https://golang.org/cl/102459 mentions this issue: io: document that ReadAtLeast and ReadFull can drop errors