I asked a Python 3 question, and tagged it python-3.x. The highlighting was messed up, because the python-3.x tag wasn't passing the lang-py
argument to the JS script that handles the syntax highlighting. I submitted this as an issue on GitHub, but it couldn't be reproduced because a mod had came along and added the python tag, which fixes the syntax highlighting.
TL;DR: If you ask a python question but only tag it python-3.x, the syntax highlighting gets messed up.