My facial expressions don't always match what I'm feeling. When I feel neutral or go into daydream mode, people tell me I look sad, upset, disturbed, or some variation thereof and when I tell them nothing's wrong, they think I'm lying and they pester me until I give them an explanation they'll accept. My younger brother is the one who most frequently does this, but my mother occasionally does this to me too. It goes the other way, too - sometimes I'll be on the verge of an absolute meltdown and no one will think anything's wrong with me at all.
Just about the only emotion I know how to express properly is happiness, and even then I don't always get it right.