MPASM supports three undocumented opcodes, tret, trap, and halt.
tret and trap are used under the 18F family of chips to implement debugging. I amd not sure what halt is used for.
If the following code is compiled under mpasm these undocumented opcodes can be seen in the .LST file.
list p=PIC18F2550 errorlevel -226 include p18f2550.inc org 0 tret halt trap end