That is the thing about math mistakes. They usually aren't random. There is a logic to them.
I deal with this in 2nd grade all the time. We use interactive remotes which anonymously put student answers on the board. We often see clumps of wrong answers. This is because students are making the same kind of error. The most common is "flipping". A student who encounters 352 - 159 will often choose to subtract the 2 from the 9 rather than regroup (borrow) off the 10s.
What is interesting to me is how often students revert back to flipping. Even once they understand why flipping is incorrect, it can sometimes take quite a while to fully break the habit. It is not uncommon to find, on a test, that a student will correctly regroup half the problems... and then flip the rest.
Knowing something is true does not mean you will always execute that knowledge. 🙂