Regarding valid instructions that may become illegal under condition:
STOP (opcode $8E) if STOP is not enabled.
BGND (opcode $82) is another possibility for normal mode use.
From the Reference Manual: "If the ENBDM control bit in the BDC status/control register is clear, BGND is treated as an illegal opcode."
But there are many-many more from the unused opcode domain. Just look at the opcode table for any unused opcodes.
Hope this helps.