The Macintosh ROM calls are accessed with a "Trap" with a leading "A" in the trap word. If for some reason the ROM is not available, this error code will be returned. The ROM is made available through a "Trap Vector Table" accessible via a microprocessor insturction starting with the hexadecimal "A."
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The Mac couldn't match a special instruction with one listed in the instruction table.
A buggy and/or corrupted application will cause this type of error.