The Theory You Know Is Outdated

Basically, what The Theory You Know Is Outdated says is that, if you've just managed to grasp the basic aspects and start using a Physics Theory, there will always be a new Theory which is much more complicated than the one you've just learned which will encompass everything you theory does, plus a bunch of very specific cases involving particles that no one has ever seem and few people really care about. The old theory remains perfectly fine, but once it's classified as "Outdated", it just never shines again.

The Theory You Know Is Outdated is one of the most important and up-to-date laws in Physics, which is rather ironic, because as a law itself, it self breaks in a way that no one acknowledges as very wrong, but natural and perfectly believable. Imagine how people would react if Newton said "For every action, there's an equal and opposite reaction, except for your reaction from this law, which will be huge compared to my action for publishing it, so just this once I will acknowledge that my law is not correct, but from now on, it is, so go figure."