If connecting to a mongod process going down, it appears that a segfault can occur if the process goes down while negotiating a connection. Steps to reproduce are basically:

Start up a mongod instances of shutting down

Connect to the instance (may not be necessary)
Send a term signal to the instance
Try to reconnect to the instance while in the process of shutting down

Attachment only runs on linux.

Not really a big issue for the shell, though somewhat annoying, but in general may affect stability of replica failover and sharding.