That, for instance, you can't write a pseudo-medieval fantasy which features a nobleman repudiating his wife on grounds of adultery. Because women in the Middle Ages were always executed for adultery. Always. (No, they weren't. In fact, I based the story an actual incident.)
Or that the feudal system always allowed the lords to buy off their responsibility to show up in person with armed forces.
Or that trains are not compatible with peasants, because peasants only existed in feudal times.