2. They could’ve been eternally stranded in the Moon’s orbit.
They knew how to get there. Getting back, well, that was trickier. (Isn’t it always?) A central issue the crew faced was the threat of being caught in the Moon’s orbit—the astronauts would be perpetually stuck circling the Moon if the Command Service Module’s engines had failed. That would’ve meant a slow, painful death from depletion of food, water, and oxygen; pretty much all the worst ways to die 238,900 miles from the nearest hospital.
Another issue was this: upon entry, the guys could have simply burnt to death in a matter of seconds because they didn't have the advantage of the space shuttle's thermal protection system, created over a decade later. This included no fewer than seven advanced materials such as "fibrous refractory composite insulation tiles" and "reinforced carbon-carbon" (not a typo), all of which protected them from up to 2,300-degree heat.