Einstein's theory of relativity has something to do with ArcMap... at least in the abstract. The theory states that measurements of various quantities are relative to the velocities of observers. And something about time being 'warped'.
Let's say your IT group installs a new disk in your network which is thankfully larger, making your work easier. They even moved your project data to this new drive. So let's say all your data and map documents are moved from the D:\Projects to the F:\Projects. Cool!
Well, not so cool, if you haven't prepared your work for this! In fact, this process may have been more like the "Big Bang"!
Most of you already know the way to fix the red exclamation, but hey, why not make it so this doesn't happen in the first place? All you have to do is establish Einstein's Theory of Relativity before hand. (I know, I'm getting way too corny... but hey, I'm from Kansas!)
The fix: Relativity
Do this BEFORE the IT move. In ArcMap, open File > Map Document Properties...
Towards the bottom, check "Store relative pathnames to data sources".
That's it! Now when IT moves your Projects directory (which contains both your data and your map documents) from one drive to another, everything still works!
Using relative paths doesn't mean that the layers in your document will never need repairing. Even with relative paths, it is still possible for a document to be unable to find its data sources if the document or the data sources are moved to a different location relative to each other, such as to a different part of the folder hierarchy or to a different disk.
Read up in the help documentation for a few more details about relative pathnames as I didn't cover everything here.
BTW, you can set this as a default for every map document. In ArcMap, choose Customize > ArcMap Options. Check the Make relative paths the default for new map documents.
I have no idea why I even connected this idea to Einstein's theory or the Big Bang... it just sounded like fun.