Apollo 13

“Houston, we’ve had a problem”

Apollo 13 should have been the third mission to land on the Moon. An explosion, in one of the oxygen tanks crippled the spacecraft during its flight and the crew were forced to orbit the Moon and return to the Earth without landing.

The two moonwalkers would have been James A. Lovell, and Fred W. Haise, with  John L. Swigert (replaced Ken Mattingly) left in lunar orbit.

More information:

https://airandspace.si.edu/explore-and-learn/topics/apollo/apollo-program/landing-missions/apollo13.cfm