Mariner 9 and the Moons of Mars
When Mariner 9 was sent to orbit Mars back in 1971, there were NO plans to image the Red Planet's moons, Phobos and Deimos!
It took a combination of a global dust storm obscuring almost every feature on Mars and Carl Sagan's persuasions to get the JPL imaging team to get Mariner 9 to make the first close-up images of those two tiny worlds.
And going even further back, many scientists and engineers didn't think astronauts would need or want windows in their spacecraft, or that imaging other planets with deep space probes was necessary. Thankfully that attitude went away rather quickly.