> I wonder how they tested that, though?
The QA process might have been performed on an emulator that doesn't support illegal opcodes.
The QA process might have been performed on an emulator that doesn't support illegal opcodes.
Another possibility is that they used a hardware device. Perhaps something that watches the 6502 `sync` pin to know when an opcode byte is being read, and verifies that the data bus contains a legal value.