nuclear physics - Do some half-lives change over time?

I was recently doing some physics tuition on radioactivity and the student claimed her chemistry teacher had said that radioactive substances can be grouped into two divisions: those whose half-life is constant and those whose half-life changes over time.

I had never heard of this before and can't think of any reason why a half-life should change, so does anyone else know anything about this?

(I know some half-lives can be altered under certain conditions, but I'm talking about a natural change over time).