"Hey, this looks a little funny, any ideas what you could do to clean it up?" is ambiguous and doesn't help you.
"Hey! This is broken. It doesn't do X, Y, or Z. Please make sure to check code thoroughly (refer to $DOCUMENT) before committing." is direct, tells people what's wrong, and isn't hostile. People will be offended by it, but those people can be made to understand that they are being given useful direct advice.