When people insert apostrophes before the "s" in plural words (example: apostrophe's), it perplexes me. Not the fact that they do it, but the immense number of people who do it. As if there is some faulty English teacher somewhere, who is teaching all those people incorrect English like Monty Python's Hungarian phrasebook.
Another very common mistake is the misuse of spaces with punctuation. It does not apply to hand writing, but it is part of keyboard writing grammar: the space goes only and always after the punctuation mark and never before, unless you are writing in French.
There is another thing that makes me cover my face with a palm, and it concerns Comms Channel post style. It is when players sign audio-visual transmissions. How do you sign your name on a video? Do you say it out loud when you are finished telling the message? Do you make moves in the air with your finger in the shape of your signature just before the camera stops recording? Signatures are for textual messages. In audio-visual, your character should present themselves verbally instead, usually at the start of the message.
PS: I almost forgot. This is something a certain person always does, despite me having mentioned it so often. There is no double Past Simple in negation in English. The form "did not talked" does not exist. The correct form, if out of context, is "did not talk". Only the auxiliary verb "do" is turned into Past Simple, if the action is in Past Simple. If the action had happened before something else happened, both actions in the past, then you will use Past Perfect, in which case "had not talked" is the correct form. Never "did not talked", never. Example 1: "She did not talk to him." Example 2: "She had not talked to him before he invited her to dinner."