little knowledge is a dangerous thing, a
Also, a little learning is a dangerous thing. Knowing a little about something tempts one to overestimate one's abilities. For example, I know you've assembled furniture, but that doesn't mean you can build an entire wall system; remember, a little knowledge. This maxim, originally a line from Alexander Pope's An Essay on Criticism (1709), has been repeated with slight variations ever since. It is still heard, although less frequently, and sometimes shortened, as in the example.