>My former manager once told me, that two most common mistakes native speakers do are "it's/its" wrong usage and using word "irregardless" <g>
>
>I noiticed recently that my son's teacher spelled "yogurt" wrongly in the paper distributed to homes...

My friend's daughter (back home) wrote "helihopter" in her homework. Parents noticed, and she almost burst into tears: "I know, it's helikopter, but I must write it like this because teacher does so".