"My favourite example is that of the naval variant of the Phantom Jet fighter. With the invention of air-to-air missiles, the designers thought that they could design a fighter plane without a cannon, as all engagements would be beyond visual range. So they built the planes without guns and they were sent to Vietnam.
"Slight problemthe missiles were not as accurate as the designers had hoped, allowing the 'technologically inferior' enemy to engage at close range with cannons, and the enemy could sneak up behind the Phantoms undetected as there was a nice rear radar gap and engage them in dogfightsnext problem, as well as being inaccurate, the missiles were next to useless at short range, so pilots were left completely defenceless in a situation that the developers had written off to the history books. The planes were withdrawn from the conflict until a cannon could be retrofitted to the design at great financial cost (it also compromised the design). Funny enough, today, all fighter jets still have cannons with their missile loads. Disruptive technologies are all well and good, but assumptions can kill."