Was There Every an Apollo 18?

No you say? Just some science fiction movie maker's fever dream! Well as Xnewsman points out that if we want to get very literal there was indeed an Apollo 18 flight.

In July 1975 there was indeed an Apollo mission. It was the last Apollo mission and the last manned US space mission until the first Space Shuttle flight in April 1981.

This last mission was called the ASTP and it was the first joint U.S./Soviet space flight.

From the WIKI

• The mission included both joint and separate scientific experiments (including an engineered eclipse of the Sun by Apollo to allow Soyuz to take photographs of the solar corona) and provided useful engineering experience for future joint US/Russian space flights, such as the Shuttle–Mir Program and the International Space Station.

Ultimately though it's main purpose was symbolic; the ASTP was a symbol of that the two superpowers were trying to get along. But it did prove useful to understanding how the two very dissimilar systems could be harnessed to work together.

It was also the only space flight taken by one of the original Mercury 7 astronaut candidates. Deke Slaton was removed from consideration early in the Mercury selection due to a heart irregularity. Not to say that there still wasn't some politics going on. Jack Swigert was removed as a crew member for just such political reasons. I would suggest looking up the Apollo 15 stamp scandal.