design fault?
hindsight is a wonderful thing.
if you were going to design a large machine today, which you wanted five-9s reliability on, would you put mission-critical triple-modular-redundant circuit boards under a drip-tray?
argument 1) you can bet sod's law means no matter where you put it, poo will settle there sometime, for some condition of age/manoeuvrings/circumstance
argument 2) deliberately invoking the god of gravity seems to be taking liberties with foreseeable problems
argument 3) how come the TMR circuit has all three redundant circuits on the one board? isn't this a common-failure which means it either votes 2 out of 3 or fails, but cannot actually survive a board failure?