Sometimes, it could be a mental block.

When I was in 5th grade, a classmate (Joe) answered that New Mexico was the capital of Mexico. He did so on a following quiz, after which the teacher corrected him in class for a second time. He got the same question wrong by answering "New Mexico" on a test a week later. The funny thing was that in the middle of the test, the teacher announced, "Joe, the capital of Mexico is NOT New Mexico."