This is not to say that some good subs don't get blindsided now and again, but the significant majority of problems that I've been witness to occur because the sub isn't bothering to screen any further than his or her own very immediate desires. No one spends time to get to know each other, the sub goes into the relationship with the goal of having their own desires fufilled and then wonders why they were treated badly by a Dom looking to get only theirs fufilled as well.
Yes, Amber, that is the other side of the coin. It's not just little innocent subs being picked off but subs who are far too ready to give all when the relationship is hardly established. (It's a power exchange, so don't give away something so precious until you can be sure it'll be cherished and valued.)

Still, supposing that the sub is trying to forge a genuine relationship, what pitfalls should she look out for?