When you connect to a computer on the network, by default it will try to connect using the "Guest" account which, if enabled on the remote computer, will allow you to view public resources such as the Public folder in users' home folders. Sometimes, and especially after running certain programs, you may see the computer try to connect using a bizarre username besides "Guest".
Apple discussion board member "tommitek" writes:
"I have a pair of Xserves and several iMac clients. File sharing is enabled on the Xserves and has been working properly. Recently one of the people here was experimenting with VNC which is enabled on a MacBook Pro. He was browsing around the network with VNC and tried to connect to the Xserves. Ever since he did this, Finder sessions from all of the iMacs default to "Connected as: VNC" with "Share Screen..." when using the "Connect to Server" (screen sharing) option instead of "Connected as: Guest" with "Connect As..." (file browsing) option when connecting to the Xserves. This only happens when connecting to machines which were "browsed" with the VNC client."
The credentials for logging into servers for the system are "Guest" by default, unless you specify otherwise using either the "connect as" button or a third-party networking program. With this option, you can save the authentication information in the keychain of the account being used, which will then be used by default whenever you access a server in the future.
There are several approaches to solving this problem:
Use separate accounts
If you have client computers in a server environment, you can set up the open directory on the servers to manage the client computers and the account information they hold. As such, one user who uses a utility that changes items will only alter his account and not the others in the system. If you have only a few users, you can create local accounts on each system instead of using a server.
Delete keychain entries
Regardless of the account setup, you can clear the problem by removing keychain entries that pertain to the server being contacted. Open up "Keychain Access" and do a search for the name of the server being contacted in the "login" keychain. Then remove entries such as network, application, and internet passwords that pertain to the server. Specifically, look for one that has "NetAuthAgent" as the access control and in the "Attributes" tab lists the server URL.
Remove duplicate keychain entries
If you would like a specified username to be used when accessing a shared folder, and no matter what username you supply (even if you specify to store it in your keychain) the system defaults to another username, you may have more than one keychain entry for the resource you are accessing. The system will only use one of these keychain entries, so be sure to remove duplicates and maintain only one entry for each network resource.