Engineers have created a device that would find out any disease or other inconsistencies a human can have, like AIDS, a broken leg, amblyopia or even psychological problems, with 100% success rate.
However there was one "bug" that kept on occurring every time again. The device always showed an inconsistency inside the brain, marked as unknown. This bug couldn't be fixed or removed and doctors never found out what inconsistency the device could mean. Since everyone, no matter how healthy they were, were marked with this inconsistency in their brain but the device worked perfectly fine otherwise, people began to ignore it and label it as an deep coded bug that would be to much of a hassle to remove.
A boy was born, it was a completely normal birth. However, after a week or 2 he began rapidly changing between two phases. He either was completely silent or struggled and screamed as if he was being burned alive.
The following 2 years of his life, he always acted strangely, doing weird movements and random outbursts of screams. His every movement was unpredictable. The parents, though, thought that it was normal as he was still a baby. He was their first child and they did little to no effort to do research about how a normal baby acts.
The boy had no friends, as he randomly hit other children in the face or bit them and then laughed. It was around that time that the parents noticed that their child might not be so normal but they marked him as special and left it be.
Until his 5TH birthday.
His parents bought him a kitten, maybe 3 weeks old. The boy ran towards the kitten. He did not pet it though or hug it. He broke its neck without hesitation and threw it in a corner afterwards. His parents were shocked and finally decided to go to the doctor. The doctor used the device on the boy to find out what kind of problem he has. After an hour of seemingly endless waiting the test was over. The parents looked at the results together with the doctor. The results weren't satisfying though.
The device showed that the boy had no inconsistencies at all.