1. Anything that requires heavy wizardry or black art to parse: core dumps, JCL commands, APL, or code in a language you haven't a clue how to read. Not quite as bad as line noise, but close. Compare casting the runes, Great Runes.
2. Special display characters (for example, the high-half graphics on an IBM PC).
3. [borderline techspeak] 16-bit characters from the Unicode multilingual character set.