Mojibake

Mojibake (文字化け, "character transformation"), is the presentation of incorrect, unreadable characters when software fails to render text correctly according to its associated character encoding. In fiction, this is used to indicate that the computer is having serious problems.

In fanfic, this could also result from the file being saved improperly, not opened properly by your own software. This trope is based on a real life phenomenon of incorrectly decoded text, after all.

Matrix Raining Code displays mojibake normally; it only has the problematic connotations when it suddenly turns a different color.

See also: Wingdinglish (language is correct, font is weird).

Video Games

 * The Marathon Trilogy uses this extensively to denote the rampancy of the AIs.
 * In Freedroid RPG hacked robots ordered to execute hcf (as in "Halt and Catch Fire") sometimes say garbled error messages.

Web Comics

 * Schlock Mercenary had a robot hacked while talking descend into gibberish.