It's vs. its. It's is a contraction of "It is," and its is the possessive. I see "The dog crouched over it's bone" all the time and it makes me want to scream.
For that matter, apostrophes in plural words. I once saw a professionally made sign that read, "Familie's Welcome."
Some of my mom's malaprops used to drive me crazy, but if you laughed you were doomed. I've caught myself using a few of them, making Mr. Sirius laugh. One I came out with just a few days ago was, "It doesn't make a hill of beans bit of difference." Mr. Sirius looked thoughtful and said, "I don't think I've ever heard that before." I told him it was one of my mom's malaprops, and he nodded in understanding. Another one for when someone did something suggestive was, "Right out in front of God and everybody!" She said this when we heard the song "My Ding-A-Ling" the first time. Mom would say things like, "She was as white as a sheep!" We never knew what she'd come up with next.
* If "malaprops" isn't the correct term for my mom's mangled sayings, please let me know what is correct. Thank you.