There is a joke among teachers that every science class begins with the disclaimer, “Everything we taught you last year was a lie. Now you will learn the real truth.” That is, the way to understanding is not to start, as would be logical, with the most current, complicated, and general cases (Let’s say, Quantum Chromodyamics and General Relativity in first year physics), but with simplifications and approximations. These can later, hopefully, be replaced with a more sophisticated understanding. A good example is the effort a teacher expends to explain to his or her general physics students that what they think is “centrifugal force” is really “centripetal force.” Once disabused, however, those students that go on to advanced mechanics will find out that they were right all along, but for a different reason.
For an explaination: http://www.explainxkcd.com/wiki/index.php/123:_Centrifugal_Force
Another instance that came up recently is the definition of entropy. Often, it is enough to say that “disorder always increases,” unless the students in the physics class are also taking biochemistry at the same time, in which case, it is preferable to say something like, “A spontaneous reaction requires that the change in the Gibbs free energy be negative, and non-spontaneous reactions can still occur with a large enough net influx of energy.” This is not as catchy, but prevents situations like this: