If you run "mongo foo", the argument "foo" could be either a hostname or database name. For example:

mongo db1-1.songza.com # this is a hostname to connect to
mongo db1-1 # this was intended to be a hostname, but....

In the example above (a mistake I've made numerous times), if the name resolver search path isn't set up right, "db1-1" doesn't resolve to a hostname, so the shell assumes it's a database name. The fact that the database doesn't exist is not an error, so you just get:

connecting to: db1-1

which looks VERY much like what you expected, but it's not. You're connected to localhost.

It would be better if the syntax was changed to be unambiguous. For example:

foo/bar ==> database bar on host foo
foo ==> host foo, no database specified
/bar ==> localhost, database bar

would eliminate these types of easy-to-make, difficult-to-detect errors.