How NASA's Apollo 14 Fixed A Critical Problem Using 'Keyhole Rocket Surgery'

Fuel sloshing around in the tanks of the Apollo landers caused all sorts of problems, including early activations of the low propellent signal on Apollo 11 and 12. For Apollo 14 an upgrade to the fuel tank was added to lessen the problems and make the flight smoother, but this had to be installed through a tiny 6cm hole in the bottom of the already made tank.

References:
http://www.aiaahouston.org/Horizons/Horizons_2013_05_and_06.pdf
https://ntrs.nasa.gov/api/citations/19710028365/downloads/19710028365.pdf
https://history.nasa.gov/alsj/a11/a11.landing.html