Yeah, there are counterexamples, but the only way to know is to read the comments or source code of the function you're calling. (T, err) doesn't convey any useful information and, in the overwhelming majority of cases, err != nil means T is a meaningless default value that should be ignored or a null pointer.
By and large I think the stuff in this repo is too much and doesn't fit Go. I don't particularly want Go to pretend to be functional, but Either and Option at least would be nice to have in the stdlib and help prevent this exact issue where there are rare exceptions to normal practices. I don't see them getting widespread use without being part of the stdlib though. If Either/Option were common in Go but io.Reader was one of the few APIs returning (T, error), that would convey a lot more information.
By and large I think the stuff in this repo is too much and doesn't fit Go. I don't particularly want Go to pretend to be functional, but Either and Option at least would be nice to have in the stdlib and help prevent this exact issue where there are rare exceptions to normal practices. I don't see them getting widespread use without being part of the stdlib though. If Either/Option were common in Go but io.Reader was one of the few APIs returning (T, error), that would convey a lot more information.