The word "мужчина" is feminine by appearance (the "а" at the end), but refers to a masculine thing ("man") an is grammatically considered to be masculine. But it's genitive singular form is "мужчины", so it is declined like feminine nouns. Yet it's adjective takes a masculine form: "высокого мужчины" instead of "высокой мужчины".
Are there any other irregularities where grammatical gender doesn't correspond to biological?
I also often come across things like "мужчинам", which is clearly a masculine declension. What's that all about?