"Grade E but edible meat" served as popular national chains is an urban legend through and through.
Let's think about this logically for a second. Usually when you hear the tale, the meat in question is labeled something like "Grade E but edible," "Grade D -- Fit for human consumption," or in the more outlandish yarns, something like "Grade F -- not suitable for human consumption." The problems with this are twofold, but let's start with the more obvious one: 90% of the time, when there are this many different versions of a story that "really happened!" it's an urban legend. As more and more people retell the story, it changes. Compare the Oral Tradition. Thanks to the power of the Internet, though, we can more easily track a story as it changes.
But it could still be true, right? Well, why would there be qualifiers for meat with its grade so clearly labeled? This means either that some Grade D/E/F meat is edible and some isn't (so why have the letter grades at all?) or that someone feels the grade needs explanation, and isn't it a little late to learn what that grade means as you're unpacking the crate?
Well, perhaps the biggest problem is that if you check with our friends the USDA, there is no Grade D, E, or F meat. Only wholesomeness grading is mandatory, whereas quality grading is optional. Any meat that has been stamped wholesame is safe to eat anyway, but just for the sake of completeness, let's go over quality grading some.
The only meat the USDA grades with letters for quality is poultry. The only grades are grades A, B, and C. Quoth the USDA:
Grade A is the highest quality and the only grade that is likely to be seen at the retail level. This grade indicates that the poultry products are virtually free from defects such as bruises, discolorations, and feathers. Bone-in products have no broken bones. For whole birds and parts with the skin on, there are no tears in the skin or exposed flesh that could dry out during cooking, and there is a good covering of fat under the skin. Also, whole birds and parts will be fully fleshed and meaty.
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Grades B and C poultry are usually used in further-processed products where the poultry meat is cut up, chopped, or ground. If sold at retail, they are usually not grade identified.
Beef is graded by age. See USDA Grades of Beef. Grade E beef is more than 96 months old and you'll only find it in canned stuff. If you're buying canned stuff I doubt you have too many hang-ups about low-quality foods. But if it's inedible, it won't be in a crate with a maturity grade. Grade E beef is universally safe to eat, and scare stories about it depend on you associating it with inedibility. Don't.
So there you have it. Now you know that whoever tells you about the low-grade meat is lying. It's easy to see why this tall tale is so popular though, as it plays on people quite effectively. One of the greatest fears of society, is not knowing what you're eating. How many horror movies, folk tales, and books tell of people unknowingly eating human flesh? The meme can even be found in the ever-popular "Little Red Riding Hood." People also tend to distrust corporate and government monoltihs. A cute story, but one with little basis in reality.
Sources: http://www.fsis.usda.gov/OA/pubs/ingrade.htm