The legendary investor's warning is silent but unmistakable.Some railroad crossings have gates that block drivers from going through when a train is approaching. Others have flashing lights to alert drivers that a train is coming. They could drive on, but doing so is risky.The stock market doesn't have the equivalent of a gate that prevents investors from losing money when danger lies ahead. However, I think it does have something similar to a flashing light. His name is Warren Buffett.Buffett has studied t ...