It's vs. Its

Evan Morris, The Word Detective, notes that at one time "it's" was considered the possessive rather than the contraction of "it is":

Until the 19th century, in fact, "it's" was usually considered the possessive of "it" -- in the Fall, a tree shed "it's" leaves. The usual contraction of "it is" was "'tis." Only when "'tis" came to be regarded as an archaic form in the 19th century did the use of "it's" as a contraction of "it is" push out the use of "it's" as a possessive. I know this is a bit hard to follow, but the point is that the "rule" used to be the exact opposite of what it is today.