I recently had to do a few not-quite-trivial things with the Jinja2 templating engine, and the more I use it the more I like it.
Github’s web hooks make it surprisingly easy to write commit triggers.
Python’s behaviour with regards to destructors can be a little surprising in some cases.
Python’s simple scoping rules occasionally hide some surprising behaviour.
It’s possible to manage multiple subprocesses in Python, but there are a few gotchas.