Speaking as someone with an Electronic Engineering rather than Computer Science degree, what is the one bit of computer science I should know to make me a better real world programmer?

(By real world I mean something I'm going to use and benefit from in my day to day job as a programmer - for instance I'd suggest understanding database normalisation is of more practical use than understanding a quick sort for which there are lots of libraries).

If I have to choose just one bit, which is a difficult decision, I'd say go for the Big O notation. Understanding the implications of O(n), O(ln n), O(n²), O(2^n), O(n!) helps you to avoid a lot of expensive mistakes, the kind of which work well in the test environment but disastrously fail in production.