If someone uses hashtags in posts or comments, they are interpreted as headline. Here's a screenshot and a link of an example:
My suggestion would be to only parse it as a headline, if between the # and the next word is a whitespace. I've seen it that way in other markdown parsers and I think that works well.
Edit: Typo
Super easy fix. The client actually needed special logic to produce the bug described here. I added this special logic a long time ago because I noticed a lot of people omit the space when they format headers in markdown resulting in misformatted headings however it's technically the fault of the author and not the app. It's easier to have the app not correct for this.