1. Partial corruption of a text file or input stream by some bug or consistent glitch, as opposed to random events like line noise or media failures. Might occur, for example, if one bit of each incoming character on a tty were stuck, so that some characters were correct and others were not. See dread high-bit disease.
2. By extension, accidental display of text in a mode where the output device emits special symbols or mnemonics rather than conventional ASCII. This often happens, for example, when using a terminal or comm program on a device like an IBM PC with a special ‘high-half’ character set and with the bit-parity assumption wrong. A hacker sufficiently familiar with ASCII bit patterns might be able to read the display anyway.