Nowadays, I almost always finds errors I missed after I post, despite a personal proofread and sometimes even after a proofread by a colleague, and I end up making several edits before I don't find any more errors.

Most of my grammar mistakes happen when I go back and edit some sentence I wrote to simplify it or improve it. The original sentence was correct, the new sentence in my head is correct, but when I edit the original sentence I don't just remove the old sentence and write the new one from scratch, and I don't end up making all the correct changes to transform the old sentence into the new sentence in my head.

I also find that I make more spelling errors than I used to since I began studying Mandarin. I think it rewired my brain to be more phonetic, and sometimes I'll write an entirely wrong word that sounds something like the word I intended to write. It's very disconcerting because in the past I could count on my writing to be near perfect in terms of spelling at least on the first draft.