The problem is that in many cases you can never know for sure if someone is intentionally missgendering or if it was just a mistake (e.g forgot their pronouns

The problem is that in many cases you can never know for sure if someone is intentionally missgendering or if it was just a mistake (e.g forgot their pronouns or if is confusing them for someone else), which is extremely easy to do in online forum because we lack a lot of social clues present in real life that help us remind who are we talking to, like their faces and their voice; more-so because internet moderators interact with hundreds of people on a daily basis and is completely reasonable to forget some details of each conversation they participate in.