In 1990 NASA launched the Hubble Space Telescope worth $1.5 billion (16 billion rn).
After the launch, the scientist found out that there is a microscopic defect of two micrometers (a fraction of a human hair) in its primary mirror. This produced blurry, distorted images.
Even a microscopic defect is a big problem for a space telescope, so in order to fix the error NASA subsequently launched the STS-61 mission.
Basically the mission is about sending astronauts into space using the Space Shuttle Endeavour and fixing the defective optics.
Three years after its launch (1993) the astronauts successfully installed corrective optics, restoring Hubble's iconic vision.
The whole operation cost NASA 900 million to install a corrective lens in orbit to fix it.