"Grammar Fail!" wrote someone on Facebook beside a picture showing the printed words "Milk it for all it's worth." But Fiona Hanington pointed out to Language Log that it's not necessarily a fail. It's the wrong spelling if worth is the noun meaning "value", so the intended meaning was "Milk it for all the worth (= value) that it has." The genitive pronoun its is not spelled with an apostrophe; the right spelling would be Milk it for all its worth. However, there's another meaning, where worth is an adjective: it could be intended to mean "Milk it for all that it is worth." And there the apostrophe would be correct (indeed, required): Milk it for all it's worth. (English is loaded with little gotcha things of this sort, isn't it?) Since both mean roughly the same thing (they put it in different ways, but it's hard to imagine one of the meanings making a true claim where the other didn't), Fiona is right to note that this is one of the very rare cases where it's and its are both correct in the same context with the same meaning. You won't find many of those.
Curiously, another Language Log reader, Jeff Gerhard, had noticed a very similar example, also about milking. A package of almond milk from Trader Joe's bore the line "We shall milk no almond before it's time!" But Jeff thinks Trader Joe probably intended the non-apostrophe reading: We shall milk no almond before the appropriate time for that particular almond arrives. In that reading, its is a genitive pronoun, and should not have an apostrophe. (Jeff notes that the idea of milking an almond is somewhat disturbing, and I agree. Do they have little tiny milkmaids sitting on tiny, tiny stools? Where are the nipples on an almond? Never mind, don't answer that.) But there is another possibility: that they meant "We shall milk no almond before it is time for that particular almond to be milked." In that one, it's is a contraction of it is and must have an apostrophe. Once again, therefore, you could therefore go with either, since they mean roughly the same.
It seems someone else on the internet also wondered about this weird example and blogged about it: see this page. You can see a photo of the packaging there.
[Thanks to Fiona and Jeff.]