>>>I wouldn't be surprised if it's the most commonly misspelled common word in the English language.
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>>My vote would be for the use of the contraction "it's" when the possessive "its" is meant.
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>>It's amazing how often that mistake occurs even in ad copy from some fairly large companies. I'm beginning to think it may become a de facto change to the English language, partly because that usage is actually more consistent with other possessive forms ("Al's", "Tamar's" etc.)
>
>FWIW, I used to have to stop and think every time I wrote "it's" or "its" to figure out whether I needed the apostrophe. Finally, I realized that the possessive "its" follows the model of "his" and "hers." Never had the problem since.
>
>Tamar

Another easy rule is that the only time to use "it's" is when you are contracting "it is".