A little learning is a dangerous thing

Meaning

A small amount of knowledge can cause people to think they are more expert than they really are.

Origin

First used by Alexander Pope (1688 - 1744) in An Essay on Criticism, 1709:

"A little learning is a dangerous thing; drink deep, or taste not the Pierian spring: there shallow draughts intoxicate the brain, and drinking largely sobers us again."