Currently, if you try to connect to an Atlas cluster with the mongo shell and an error is encountered, the shell spits out the following before the error is logged:
*** It looks like this is a MongoDB Atlas cluster. Please ensure that your IP whitelist allows connections from your network.
This can be helpful in some situations, but it can also deter a user from seeing what the actual error was (eg. authentication failure).
This messaging was added in SERVER-38625, and perhaps could use some improvement such that it is only displayed when a) the target is an Atlas cluster AND b) the cause of failure is due to a connection timeout.