The CMAP spec is a little bit light on tests around the behavior of failed attempts to create a connection. For instance, the "checkOut" pseudocode indicates that a connection should be created if totalConnectionCount < maxPoolSize, but its easy to miss that a ConnectionCheckOutFailedEvent should be emitted if that connection fails to connect. 

It seems we could generally improve test coverage around the creation of connections. It might be enough to add a flag to the checkOut test runner operation informing the runner that the operation should fail with a specific error type. If the checkOut fails with failToCreateConnection, then we should expect a ConnectionCheckOutFailedEvent.