Prof spends 18 years on experiment that goes to Saturn, somebody
forgets to turn it on:


Not the first time someone screwed up. Seems that weeks after
Voyager 2 was launched it was expecting messages from JPL.
Hearing nothing, it figured that its primary command
receiver was broken, and switched to a backup receiver.
Well, the problem was that the expected message was never
sent. There was a risk that if they allow Voyager to switch
back to the primary receiver the switch circuit might break
and then become completely deaf. Later the backup receiver's
automatic frequency control circuit fails (a filter loop cap
shorted to ground it seems).

But space probes are designed to still be able to work and
get at least some data back if some screwups happen.