Flask-Markdown bug
I encountered a mysterious bug in Flask-Markdown: footnotes in one document were polluting others. I noticed footnotes being duplicated in a markdown-formatted sidebar, and then I found them showing up on new pages altogether.
The (approximate) cause was obvious: markdown was keeping state between different renders.
It turns out that Python-Markdown allows you to instantiate a renderer with options set and reuse that, if you choose, and Flask-Markdown takes advantage of this. However, when using Python-Markdown in that way, you need to reset() the instance between renders.
Solution: change line 69 of Flask-Markdown’s markdown.py from:
return Markup(self._instance.convert(stream))
to
return Markup(self._instance.reset().convert(stream))
Simple.
Unfortunately, it looks like the maintainer of Flask-Markdown hasn’t been active on github in quite a while, so this problem may persist.