Did you ever get a feeling about a person that you couldn't trust them, but you just don't know why?

Did you ever meet someone, for example a colleague or friend of a friend and for some unexplained reason, develop a funny feeling about them? You just felt that you couldn't trust them, something about them made you wary and you just couldn't put your finger on it? They come across as perfectly normal and because of the way you became ociated (eg. a friend of a friend, or a close work colleague) you would imagine that they are someone you could relate to or at least not have suspicions about but something about them makes you stop from becoming more close to them. Why is this?