Linear algebra purely as row manipulations. I've <a href="http://numberwarrior.wordpress.com/2009/06/23/matrices-should-not-be-taught-at-the-high-school-level/">written about this here</a>:

> Students stuck in a rut of thinking of
> matrices as a clever way to arrange
> numbers will get lost and confused; I
> know this because I was one of those
> students. I had to “de-program” what I
> was taught in high school before I
> could grasp what was going on.