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.
Best Answer
There is a fairly well-known fable about a centipede who was asked how he could keep his hundred legs moving in the correct order when he walked - and as soon as he thought about it, quite lost the ability to walk. This blog post is one of many retellings of the story I found on a quick internet search. That's a close parallel to the Hungarian story about the smith.
So you could refer to "the centipede who forgot how to walk when someone asked him how he did it". However the centipede story is not so famous that it has given rise to an idiom of its own in the way that "cataract-smith" has become an idiom in Hungarian. I can't think of a one or two word translation for "cataract-smith"; you would have to explain a little more.
After writing the above I did another internet search and found out that there is a quick way of referring to this story: according to this Wikipedia entry it is known as "The Centipede's Dilemma", and the story originated with a particular poem published in the 1870s rather than being an old folk tale as I had thought. But I must say that I had not heard the specific phrase "The Centipede's Dilemma" before today.