There is a Hungarian idiom "cataract-smith" (hályogkovács) which means a person who is instinctively very good at a complex task, without formal learning, being successful either because of being good at self-learning or having a good instinct. However, if someone tried to teach that person in a formal way, it would actually hinder, not help. This last issue is also an important part of the idiom.
The source is a story of a common village blacksmith from the 19th century, who, despite never having studied medicine, was very good at operating on cataracts. He got so famous among the common village folk that they went to him instead of real doctors for eye surgeries. Once some famous eye-surgeon heard about his story and was curious to test him (or, in a different version, there was a patient in a condition even the most experienced surgeon didn't see any chance for a successful operation). So, the smith came up to the patient, witnessed by experienced eye surgeons, he took out a rusty pocket knife, and removed a very difficult cataract (regarded as impossible by the doctors) very easily in just a few quick hand movements.
One of the doctors was horrified by this, and got angry how careless the blacksmith was, and started to explain how delicate the eye is, showed pictures of the interior of the eye, showed where the nerves are, explained how sensitive they are, how just the slightest mistake can make the patient permanently blind, showed what kind of fine, delicate scalpels the real doctors are working with (instead of a dull, rusty pocket knife), and scolded him for how reckless he was. The blacksmith, who've never seen such magnified images of the interior of the eye before, was mesmerized by it. He then tried to proceed with the other eye of the patient, which had a very mild cataract, so simple even a beginner surgeon could easily remove it, but his hands were trembling so much he didn't dare starting the operation, and he never touched a cataract ever again.
Edit: I've been thinking about its more common usage today, and it usually implies doing a risky job without any formal training, and succeeding while others are doubting success.