I can glance at a document, such as a letter or an email, and immediately notice any errors related to punctuation, alignment and capitalization. For example, I can discern the difference between a comma and a period simply by putting the document in my line of sight, and understand whether that punctuation mark is used correctly. Regarding spelling, I can do a quick visual scan over each paragraph and notice if a word is misspelled, even before I understand the context in which the word is being used. Once I settle down to read the sentences, I find all grammar errors, as well as any other errors my quick glances didn't catch. I could be a copy editor's dream.
In a previous job, reviewing my colleagues' proposed letters actually became of one of my job requirements, and I loved it. I knew I had this special but inexplicable talent, and that I could show off that talent to others, particularly to my boss who was responsible for bonuses and promotions. 
_________________
DSM-5 Diagnosis: Autism Spectrum Disorder, Without accompanying intellectual or language impairment, Level 1.