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Derailment
From SnesLab
Derailment is a failure state the 65c816 can enter. Usually as the result of a programming error, derailment from the instruction stream occurs when the CPU misinterprets an operand byte as an opcode byte or vice versa.
To safely shut down the CPU, consider STP.
Zeroing memory can help mitigate derailment because accidentally running opcode 00h causes a BRK interrupt.