The hint NOP opcode 0F 1C /r with a memory operand is CLDEMOTE, a memory-only instruction the toolchain's own table carries; the decoder rejects the encoding instead of naming it. The rejected-encoding side of the supplementary table names it from the bytes, and the corpus row 0f1c03 pins the text in the unlisted fixture, round trip byte exact. Assisted-by: GLM 5.3 Flash