So maybe I'm exaggerating a little (I'll admit I am prone to hyperbole), but I am finding the more I work with students of all ages, the single most important technical skill that they absolutely need to understand is the difference between Save and Save As....
I honed my teaching of this particular concept with first graders, so I figure if they can get it, so can everyone else. Files at school are damaged, corrupted, or lost all the time because of a general failure to understand what happens when you start creating multiple copies of things. And saving them in multiple places. Especially if the server comes into play. "The computer ate my work" or, "The computer is broken!" almost always stems from a user error while saving.
In general, I drill ⌘S into my students' heads. The first time one saves a file using ⌘S, the computer automatically does a Save As, which gives them the opportunity to 1) name the file and 2) tell it where to go. From that point on, they will never need Save As again, UNLESS THEY WANT TO CHANGE ONE OF THOSE TWO THINGS! ⌘S will do for the rest of eternity. So how do I test them to know if they really know the difference? I ask them all time! Should we Save or Save As? How about now? Why would you choose Save As over Save in any circumstance? If it's a simple matter of changing the name of a document, I teach them how without ever needing to open the application.
Now that we have Tarrier Apps, this skill isn't quite as crucial when it comes to word processing. However, the concept still applies in every other application students may use...Comic Life, Photoshop, iMovie, Geometer Sketchpad, etc. Recently, the Save/Save As conundrum got a few middle schoolers stuck in the middle of a video project. When it comes to movies, that's a big deal, because source files have to live in a certain location, and if you tell your computer midway through a project that it's all moving somewhere else, you get the lovely spinning beachball of doom and your work is lost. Of course, at the heart of Save vs. Save As is a deeper understanding of file structure. I start with the basics in 1st grade because they're just not going to get that part yet, and they simply need to know what to do and how to do it. Eventually, with repetition and experience, the deeper understanding will come.