Myth: You can only absorb 30g of protein in a single sitting.
This idea probably stemmed from supplement companies and the fact that a typical serving of protein powder is around 30g. Gym lore have seemingly kept this myth alive over the years, and it has spread into the popular domain as well as sport.
Coincidentally, research over the past 8-10 years has instilled a grain of truth to this concept since rates at which muscle growth can occur in response to a single protein meal are 'maxed out' with a 20-40g dose of high-quality protein. That said, you will absorb almost all of the protein you ingest, regardless of how much is in a given meal (see table in the image). If you eat a big steak containing 70g of protein, you'll digest and absorb pretty much all of it.
This doesn't mean that anything over 30g in a single sitting is wasted because it isn't used towards muscle protein synthesis/building muscle. The remaining digested amino acids will continue to trickle into your blood stream, and might help with minimising muscle protein breakdown (the other half of the protein balance equation). Some of these amino acids will be converted to glucose for energy. Also, protein is the most satiating macronutrient, meaning it will keep you feeling full for longer, which is particularly important for those trying to drop a few pounds.
This myth doesn't even pass the reality check. With no fridges to store meat and the fact that our ancestors probably ate when they could, the human race would have become extinct a long time ago if this were true.