I know this says 'as a child' but for me, basically everything I believed before I was 30 turns out to have been false:

• government is good for you
• the liberals are the tolerant/accepting ones
• conservatives are not
• science is always right
• you must have a college degree to succeed
• having kids is cancer
• sleeping around is great
• the world is going to end due to climate change
• adults are in charge of the world
• alcohol is great
• cigarettes are great
• the talking man on TV is always right, and knows everything
• if it says so on the box, it must be true
• my parents know what they're talking about
• America is the 'free-est' nation in the world

plenty more I just don't wanna bore anyone...