Things were going well, as we'd come to expect with Apollo missions, when the deceptively understated message "Okay, Houston, we've had a problem here..." came crackling over the radio from the Apollo 13 Command Module "Odyssey" over 320,000 kilometers from earth, and an oxygen tank exploded, severely damaging the Service Module and leaving the Command Module without power or air.
Things were touch and go, and it was decided to move the three astronauts into the Lunar Module "Aquarius" that was only designed for two, and attempt a tricky loop around the moon using Aquarius's engines that had been intended for the lunar landing. The Lunar Module, of course, was not equipped to handle reentry into the earth's atmosphere; the astronauts would have to use the battery-powered Command Module for that.
There was seldom a more nervous moment, but aside from the cold, cramped Lunar Module and the very short rations, the flight ended very well and proved that the Apollo system could weather some pretty heavy duty malfunctions and still make it through.
Interestingly, the splashdown was one of the closest ever to the recovery ship, almost hitting the deck.