1.The Roman Empire fell because the Late Roman Army was weak/"barbarised"/didn't wear segmentata and fight with the gladius (cos they rock and are cool and stuff!)
2. The Roman Empire fell because of Christianity/"corruption and moral decline"/lead pipes/anything other than economics and civil wars.
3. Christianity caused the Dark Ages by crushing reason and burning all books and scientists.
4. People in the Middle Ages didn't wash regularly.
5. Period X was "bad"/ Period Y was "good".
6. The Crusades were mainly motivated by greed and politics rather than religion.
7. The Crusaders were idiots who generally got spanked by the Muslims in battle.
8. Medieval armies were made up mainly of peasant levies, were led by knights with no sense of group tactics and had no discipline, drills, command structure or co-ordination.