What exactly is a bogus key, and how to treat it correctly?

I am currently trying to set up opendkim mainly as a verifier and have come across the following passage in the man page for its configuration file (man opendkim.conf): > BogusKey (string)
Instructs the filter to treat a passing signature > associated with a bogus (forged) key in a special way. Possible > values are neutral (return a "neutral" result), none (take no > special action) and fail (return a "fail" result; this is the > default). I have thought a while about that passage, but I don't get it. What exactly is a "bogus key" in that context? After all, a signature can only pass if it has been encrypted with the one private key which matches the signing domain's public key, which in turn is queried by the verifier upon verification. So how can a bogus key be associated with a passing signature?