[Programming with Objective-C: Dealing with Errors](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/ErrorHandling/ErrorHandling.html) has more ambiguous wording:

> When dealing with errors passed by reference, it’s important to test the return value of the method to see whether an error occurred, as shown above. Don’t just test to see whether the error pointer was set to point to an error.

It's also relevant that your quoted excerpt says "in the Cocoa error domain." I strongly believe we need to be defensive here and not assume that a `nil` error implies one didn't occur.