The ninth commandment-- you shall not commit false witness against your neighbor-- doesn't have the attention-grabbing sex and/or violence mentioned in a lot of the other commandments, but if people kept all the other commandments and were habitually sloppy about the ninth, it would be a misery to live with them.
Fictional possible counterexample: Hobbits-- they have a strong tendency to malicious gossip, but they don't seem to punish each other much, so their culture is tiresome but not actually horrendous.
It's a tricky commandment-- how careful do you need to be to avoid breaking it? You've at least got a decent chance of telling whether you're lying, but what can you believe when other people say it? Still, it's important.
Link thanks to supergee.
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