Q:

How do I diagnose problems with a fresh install/update of a TeX distribution?

Here's the (hypothetical) scenario: I've updated my TeX distribution in some fashion.  Now all of a sudden, a package that used to work just fine is no longer working.  Of course, the list of what could have gone wrong is endless, but I would expect there to be a few common things that one should check first before trying to dig too deeply.  So that's the question:
What are the common causes of a package failing to work after an upgrade, and how does one diagnose and cure them?
After all, one wouldn't want to look stupid on a prestigious question and answer site by asking a dumb question, would one?

A:

The first thing to do when updating is to make sure you have a backup. Since TeXLive installs each year's version in a separate folder it's easy to have your old distribution available.
You should always check your local texmf directory for old copies of things that are normally in the main texmf tree, but you may have added manually at some point in between updates. (This is especially true for large packages that issue development builds but don't update on CTAN very often.)
It's helpful to join or restart your subscription the TeXLive Mailing List (at least right after updates), as this will alert you to ongoing issues in real time.

