I asked a question, Have we seen the last significant experiments in the configuration of jet airliners?, and then by chance while reading something else came across a link to How the 747 Got Its Hump, an article describing various key developments of aircraft design that can be attributed to unpredictable events and causes.
If we consider only airliners, there are some obvious drivers of design in aviation: economy, safety, performance, passenger comfort, reliability, ease of maintenance and so on.
There are some though that appear to be on-off or wholly unexpected factors that nonetheless have left a permanent mark on design - significant random mutations, if you want a biological metaphor:
Today’s 747 has a large upstairs cabin because Trippe thought it would be a freighter, because a round hump produced too much drag, because Pan Am bosses had fond memories of the Stratocruiser’s bar (and all-night parties held by the light of the Pratt & Whitney R-4360s’ flaming exhausts), and because fuel prices went through the roof. Nobody set out to design the airplane with an upper deck. It came about because it was possible and because it adapted the 747 to a changing environment. It evolved. (How the 747 Got Its Hump)
What other notable accidents of chance have had enduring effects on aircraft design?