What's the worst mistake or faux pas you've made on a web forum, other than WrongPlanet?
Poor theory of mind has led me to make many, as I genuinely don't realise the emotional impact a post will have on others until the mistake has already been made and people are offended, and so I have to learn from my mistakes every time rather than having innate common sense to avoid them. Time and time again, the very music of my online posting has been laughably out of key with that of other members of a (non-WP) forum. It's a miracle I've survived so long on this forum, I can assure you, given what length of time and post count I have here. I know this is an autism forum, but still.
The obvious place where I've made my worst mistakes in living memory is a forum about city, country, region etc. demographics called City Data. All despite an extremely low post count there, which despite a special interest in the subject is testament that it's a forum where one would be particularly likely to fall foul of poor theory of mind, and that the forum is particularly easily offendable if your theory of mind is poor.
I'll pick as my single worst mistake there, devising and sharing with the forum what I called a Social Exclusion Index which ranked countries and states based on indicators I chose (I won't share those indicators or the results here to spare people possible offence). This offended people and the thread was closed quite quickly.