readers who know everything

One of the disadvantages of writers' group is that you get to meet the readers who know absolutely everything and are delighted to tell you everything wrong about your world-building.

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.