Not being able to stop the snowball effect. Key example is when I was playing Rome Total War as the Britons, Spain managed to conquer all of Gaul (France) and Germania (Germany). Before Spain became a super power, I was having some success in Germany conquering mostly the northern cities with ports. However, my faction leader who happened to be also my best general, didn't have any children to become the future heir. I had forgotten you need more settlements, in order to get more family members. With Spain slowly creeping into Germania and a weak family member becoming faction leader, I was forced to withdraw from Germania. Even when I had invaded Spain with three armies and took half of Iberia, they were too strong and able to take back their lost settlements. Don't get me wrong, I love a challenge and the idea of being able to lose. However, I should at least be able to claim victory from the jaws of defeat. If I would of continued the game Spain would of conquered all of Italy and won, having conquered Rome.