“What they don’t know won’t hurt them.” Or
“What you don’t know won’t hurt you.”

Totally false:

1. Knowing — if they ever find out it will hurt.  “What do you mean you’ve been sleeping with my sister for the last two years?”  (Purists might argue that once the secret is learned it is the knowledge that hurts, not the ignorance.)

2. Despairing — shit will keep on happening and you’ll attribute reasons to it other than the real cause, which you don’t know but hurts you nonetheless.  “Stu thinks his rejection letters from publishers are because he needs to rework the plot.  Really, it’s because I blackballed him as a pedophile.  He’ll never work in this town.”

3.  Ignorance — Though at times blissful, ignorance of how something works can leave a person frustrated and/or open for predators.

“I turn the key 1000 times, but the car won’t start.”

“Did you hold the brake down while turning the key?”

“Why would I do that?”

Not knowing hurts.