Memorization of incorrect math facts
The idea that long term memory cannot be ever changes is absolutely wrong, for sure some thing that is learnt and practice for a long time while being a child has a difficulty to change after while, but it is not impossible.

About learning you have to know that we have different kind of learning disabilities.


  Specific learning disorder is a developmental disorder that begins by school-age, although it may not be recognized until later. It involves ongoing problems learning key academic skills, including reading, writing and math.


These patient usually can learn about everything but for specific type they have a difficulty learning. Based on what their difficulty is, they might get different treatment. Diagnoses is not also easy in some cases, for example patient who has difficulty learning math might be because of his difficulty reading text so if you try teaching math only by images and art and symbols he can learn.

reference here.