After the command to return a lost ox, we find: "You should do likewise for his dress (לשמלתו)."
However, two pesukim later, we see וְלֹא-יִלְבַּשׁ גֶּבֶר שִׂמְלַת אִשָּׁה (and a man shouldn't wear a woman's dress) — apparently, a dress is an exclusively female piece of clothing. If so, why didn't it say in the pasuk about returning lost things "You should do likewise for her dress?"
The answer is not that it is coming to teach you that you have to return something even when done in transgression, or anything along those lines, because Sifri (224) learns from the example of the dress to include anything with symbols on it. Even if that is the case, it should have used the normal language, because it's not coming to teach you that rule.
And if you say a שמלה is not an exclusively female piece of clothing, then why does it say in the pasuk about not wearing the opposite sex's clothing "שמלה" to describe a woman's clothing but "כלי" to describe a man's?