Someone should make a set of documentation keys which are invalid, but still look valid. That way they could be used for documentation, but couldn't accidentally be copied into a production environment.
The simplest thing I can think of is by generating keys that follow the exact right structure but some invariant like n = p * q is broken. Of course that relies on the assumption that implementations do some basic sanity checks like that. I don't know if that's a fair assumption to make.