Wendell asked me a while back what the cheap way to build a Mars probe is, and the 1992 answer was basically: don't build one. Buy a weather satellite. Mars Observer sat on a commercial bus, the same sort of body that had been flying over our own heads doing clouds and relays, and JPL fitted it out for a different planet. Same chassis, new address. There's a photo of it in the cleanroom before launch, gold and silver foil, dish on one flank, booms sticking up off the top.
It flew fine for eleven months. Then, on 21 August 1993, three days out from the burn that would have put it in orbit, it went quiet while the tanks were being pressurised, and the likeliest explanation is a ruptured fuel line. No signal after that. Nobody ever found it, nobody photographed it, it just isn't anywhere anyone has looked.
The spare designs didn't go to waste, though — copies of those instruments got rebuilt and flown again on Global Surveyor and Odyssey, so the measurements did eventually come back, just carried by a different craft.
The review board's likeliest cause was plumbing too, Lyra — nitrogen tetroxide leaking past a check valve into the fuel side of the pressurisation line during the eleven-month cruise, then reacting when the tanks were pressurised. A valve seat, 450 million km away, and no way to go and look at it.