Can such a thing really happen? Perhaps before we answer the question, we need to consider the order through which people learn a new language. The process starts with hearing but not making any meaning out of it, then understanding without being able to speak, then followed by speaking without being able to read it, and finally reading without being able to write it.
To justify the above process, consider the language learning sequence of babies born without any language by default:
The baby starts to hear sounds but could only differentiate between high pitched and low pitched sounds without understanding what each means.
The baby starts to attach meaning to different speech patterns. For example, from the speech, babies can discern when one is angry with them and when one is just joking and trying to incite them to smile.
The baby starts to distinguish between voices of different individuals like the mother’s from others’ around it.
The baby starts to decode meaning of some simple expressions and acting accordingly.
The baby starts to mimic those simple expressions that it had already understood their meanings, hence adults around it could discern that the baby is trying to talk.
The above process continues to get better until the child becomes adept in verbal communication.
The child gets basic education in order to learn how read and understand what he is reading.
Then finally as the child progresses in his education, he becomes able to communicate his thoughts through writing.
The above sequence of the language learning process was further proven true by an experience that happened during one job interview.
A certain girl was very fluent in her English speaking and was nearly given the job until the interviewer asked her to write an application letter for the job she was seeking. That was where she was stunned in a deadlock. Eventually she lost the opportunity for the job.
Furthermore, it is not uncommon to see some individuals that understand all the expressions in a certain language but find it difficult to speak the same language. That is the language learning process yet to be completed. Hence, the question again: can someone write very correct English and not be able to speak correct English?