Don't fly the 139 but have a little experience with the 76. Actually flew one with the floats inflated about 100 miles after my CP accidently pushed the button that was then located on the underside of the collective. Aircraft actually flew rather well at around 75 knots.
That being said, there was a 76 that had the floats accicently deploy while the aircraft was IFR and doing around 130 kts. The crew got it under control but it was a close call. An electrical short was found on investigation.
As far as Murpy's Law is concerned, reminds me of a discussion I have many moons ago with a couple of Allison engineers. A couple of us informed them that we were flying the 76 with the electrical engine overspeed C/B's pulled. They insisted that the system was incapable of inadvertent activation.
Sounds like a scenerio that out to be practiced in the sim.