Plug in an Ethernet adapter or wake the computer around unencrypted WiFi (or encrypted WiFi but with a broadly shared password) with the same name as one the computer has used. In addition to direct attacks on anything listening for incoming network connections, I believe applications will keep running and using the network while the screen saver lock is enabled; it may be possible to inject an exploit in a server response to a request from a browser, an email client, etc. though the broad use of encryption at the application transport level makes that much less likely.