The incompetent with nothing to do can still make a mess of it. — Laurence J. Peter
Don’t try to do jobs you don’t know how to do. This does not mean that you don’t try new things or attempt tasks you haven’t done before. You are interested in extending your knowledge and skills whenever the opportunity comes along. Rather, it means that you don’t try to do things you have no reasonable knowledge of or familiarity with. You stay within your known capacity and expertise. If you are going outside of your personal and professional limits, you associate with an expert in the new area. You can learn, can be trained, but don’t act as if you are when you aren’t.