Honestly, computer literacy in general. Even back when I was in university, basic comprehension of how computers work was lacking even among IT students. Not as egregious as the various user horror stories you'll probably have heard, but bad enough.

I admit that our software dev 1 teacher starting the first lecure by opening a terminal, viming together a basic Java Hello World without explanation, then compiling and running it wasn't a great start for anyone except the lucky few that knew enough to not be deterred.

But the general idea what files are should be common knowledge, the "hacker" impression of the command line should be demystified a little, and the distinction between monitor, computer and hard drive... actually, maybe we should leave that one out for the sake of entertainment.