SSB Log Entry 409

With the way gossip works in #ssb, does the local machine ever persist messages to disk that cannot be verified? Eg:

You receive a message from peer A that was authored by peer B
The message is syntactically valid, but you don't have peer B's complete feed, so you cannot verify the message is authentic.

Do these situations ever happen in the real world? If so, how does SSB handle them? Does it:

Never happen, because SSB doesn't sync feeds like that ever.
hold on to the message until it has enough information to do the algebra on it.
discard the message entirely since it can't verify it based on current knowledge.

Still learning the protocol, so pardon my lack of understanding if this is not how SSB works.