The brain behind a Mac is a processor chip (a Mortorola MC68000, MC68020 or MC68030). This processor is driven by certain codes ( 'op codes' ), that are decoded and translated by the processor into specific actions.
When the processor is receiving a code that doesn't match the internal listing of 'op codes', an error id= 03 will occur.
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The application sent a code to the processor that was not part of its code list.