Ebert's Law as defined by Potter Sue of the Day and Urban Dictionary: When you ask somebody to try their own hand at something before criticizing your efforts, you have violated Ebert's Law and lost the argument. Roger Ebert is not a filmmaker (though he co-wrote three films with exploitation film director Russ Meyer, his most famous being 1970's Beyond the Valley of the Dolls), but he knows what he likes and doesn't, and has every right to say so. Similarly, people don't need to be chefs to recognize a good restaurant, or musicians to appreciate a symphony. Badfic writers break Ebert's Law often upon running across the PPC or other badfic sporking communities which have mocked their work.
It is still a good idea for PPCers to practice writing in the fandoms they spork badfic in, partly in order to improve their grasp of the canon, partly to improve their general writing skills, partly to increase levels of goodfic to balance out the badfic, but mostly because writing goodfic is fun.
Defined thus by Pottersues: It's a sort of corollary to Ebert's Law. If you accuse somebody of putting something down because they're just jealous of the creator's 1337 5K1LLZ, particuarly if you invoke the opinions of others who liked it in order to support your argument, you are breaking Not-Bob's Law and have lost the argument. You can think Kill Bill was pointless without being jealous of Quentin Tarantino's filmmaking. Named in honour of "Not-Bob", the nickname given to a mysterious person who trolled the Pottersues livejournal for quite a while.