The garbage one gets a
terminal (or terminal emulator) when using a modem connection with
some protocol setting (esp. line speed) incorrect, or when
someone picks up a voice extension on the same line, or when really
bad line noise disrupts the connection. Baud barf is not
completely random, by the way; hackers with a lot of
serial-line experience can usually tell whether the device at the
other end is expecting a higher or lower speed than the terminal is
set to. Really experienced ones can identify particular
speeds.