When using the Go -> Connect To Server option in Mac OS X, your computer account credentials may automatically be used to authenticate to the server. I’ve seen this to be particularly true when a user is logging in to their Mac via Active Directory credentials and they are attempting to connect to a server that is also joined to the domain.
It is possible to force the connection to prompt for a different username and password. I’ve tested this with a variety of protocols including SMB, CIFS, and AFP. I’ve read others claim this method can also be used to connect to the same server multiple times with different accounts, but I haven’t tested that.
In the Connect To Server prompt, enter the server address in the following format: smb://other_username:*@server_address
After clicking Connect, a credential prompt will appear where a password can be entered.
You can verify which usernames are currently being used for all active server connections by using the mount command in Terminal: