Keithley 616 has the MOSFET protection scheme Ed described. I checked my Keithley 196, it uses JFETs for protection diodes.
I know I am going off topic but Heathkit IO-4235 oscilloscope has a "feature" that blows up B-E junction of power supply BJT Q101(and releasing the magic smoke of couple of zeners with it) if not loaded.
Ozan
