Being a mommy. It's a constant lesson in humility.
As if pregnancy and childbirth aren't enough in themselves, then there's the infant phase, where you tend to go out in public with a variety of child bodily fluids in hard-to-see spots (whether it's because you saw said spot and didn't care or just got nailed with something and forgot it was there...).
Then your children learn to talk, and all of a sudden you're in Target and you can't stash the Tampax box far enough from your daughter to make her stop picking it up and studying it intently. "Mommy, look at this girl! She has orange shoes! What is this? What's in this box? Can I touch it? Can I open this? Is it candy? What's in here????"
And then, inevitably, she will hold the box up at every stranger you pass, making direct eye contact and shouting, "Look! It's lady things! See these?" as you go.
Just forget about trying to spell things in front of your children after the age of 4. You'll say to your husband, "I don't really care for those shoes - I feel like they make me look like a w-i-t-c-h." Then your son will shake his head sadly at you and say, "Mommy, a witch? We don't say witch..."
Humility. You might as well just learn it now.