Conditions aboard Apollo 8 were deplorable.
http://en.wikipedia.org/wiki/Apollo_8
Minor, but annoying and frequent technical difficulties made the early part of the spaceflight stressful, later came the sleeplessness out of watching for other malfunctions which might arise. Then, one of the astronauts came down with an illness, with uncomfortable, gross and distracting results.
Meanwhile NASA reacted to these developments with total apathy, and expected the crew to still make the scheduled video broadcasts, while acting as if nothing was amiss.
When they finally reached the Moon, it was difficult to observe, because of other inconveniences. Then came the journey back...
What if the crew got so annoyed and frustrated, that they decide to alter their deorbit burn to splash down in the Pacific Ocean near Vladivostok, and defect to the Soviet Union?