ConnectException: (local IP): Probably rmiregistry isn't running.
UnmarshalException/Connection reset: Probably (the remote) java crashed (dead battery?).
java.lang.NoClassDefFoundError: javax/media/Player: JMF is not installed or not on the classpath (for Webcam).
Hardware not responding: Wrong serial port settings, low battery, no serial port.
EEE doing strange things: Run chkdsk /f and reboot.