A programming can re-use bits of their code, so they don't have to write it over and over in their addon. This is good.
That sort of re-useable code can be put into a "library" that can be shared by different addons.
Idiot users are confused about installing and managing those libraries. So some addon authors "embed" libraries with their addon. The user downloads just the one addon and all those libraries come with it, ready-to-go.
The problem is when there are multiple addons all using embedded libraries. Some of those addons are using /the same library/. I hope this isn't much of a problem, and that either WoW or the libraries are smart about not doing duplication.
However, sometimes some of those embedded libraries /are different/. Maybe some are old, and maybe some have even been modified!
So imagine you have all of these addons all with their own version of that library. This is _amazingly stupid_. Each author should be updating their addon to /at least/ embed the /latest/ version of their libraries.
So what's the solution?
Don't embed libraries!
- Give a simple list of those dependencies, and link to their library-addon page.
.. or always keep embedded libraries up-to-date!
- An addon's source code repository should just fetch the current release for every library, and the author should test every new release with latest-version libraries.
How does a user recover from this madness?
Well, we can't.
Hack on each addon to link to the latest version of each library? This can cause major problems when the addon is expecting some specific library version or modification. See World of Warcraft AddOns via repositories for the nightmare I had.
It's best to bug the author about not using embedded libraries at all.