I'm especially interested in anything which >> gave you an "ah-ha!" moment when you were working with library data -- >> the implicit things which didn't make sense until you knew why those >> crazy librarians did things the way they did.
I'd add that you should be open to accepting that some of those things STILL won't make sense once you know why librarians do things the way they do.
Much of what we do indeed simply doesn't make sense -- I mean, we got here somehow for certain reasons, and understanding the history can help understand how we got to where we are, but often where we are is really really unfortunate. (And even many of us librarians don't entirely understand how we got here, if we're under a certain age!)
But anyway, you've probably already seen it, but Jason Thomale's Code4Lib Journal article might be useful for providing some of that background for Marc (although I don't neccesarily think his explanation/analysis/solutions are airtight, they're an introduction).