ConnectionPool's _hostExipration is default-constructed, which initializes it to a value of 0. As a result, all ConnectionPools begin an state in which _hostExpiration indicates expiry. Normally this isn't an issue, since a SpecificPool generally is only created when a connection is requested from it. However, SpecificPools can also be created if theyre included in another pool's hostGroup (i.e. replica set). In such cases the pool will actually transition to isExpired = true, as the event timer will fire upon creation.