I posted recently on a large subreddit to promote my app. I read the rules on the side of the page that indicated which day to self-promote and what flair to add, etc. I still got banned a few minutes later, and the post taken down.

I asked the mod why and they directed me to yet another page with a huge list of rules that weren't referenced in the self-promo rules I had seen. I understood the intent and it's their rules, but it did leave a bad taste in my mouth, having tried in good faith to follow their instructions.