A little learning is a dangerous thing
Meanings of "A little learning is a ..."
It means a small amount of knowledge can mislead individuals into believing that they are more experienced and expert than what is actually the case.
Example: "Just because she painted her room by herself, now she thinks she can do the entire house—inside and out! A little learning is a dangerous thing, I tell you."
The 'little learning' version is widely attributed to Alexander Pope (1688 - 1744). It is found in his An Essay on Criticism, 1709.