This is the mirror of the NOP instruction, where the 7 opcode bits are meaningful (the optional 16-bit integer is ignored), but it unconditionally triggers an error.
The value of this opcode (all 1s) matches unwritten E(E)PROM cells. This way, the core will do nothing if it boots from a blank Flash memory, for example. This opcode should never be used, it's only here to catch accidents/exceptional failure conditions.