-- University of Idaho professor David Atkinson, whose years of work on an
experiment to measure the winds on Saturn's moon Titan came to naught because
someone forgot to turn it on.
One of the first video cameras sent into orbit failed to show any
images. A diligent search for the problem found that no where in the
prelaunch check list did it say to remove the lens cover.
The first French Ariane 4 rocket exploded 47 seconds after launch. A
diligent search found nothing until someone noticed a worker using a red
rag to sop up a drip that continued until heat from the launch expanded the
metal to make the seal. The red rag prevented oil from dripping into a
stage one cooling fan.
Us wet ware being what we are, there are plenty of those examples around.