Something which I have very recently learned the hard way is that definitions matter. Perception matters. I have long held a personal policy of telling only the truth and not lying. However, according to at least one alternative definition of the word lie it is possible to lie with the truth if one uses it in a way intended to mislead. Most other definitions of the word lie however hold that a lie must consist of an untruth or a falsehood. I share this to again stress the importance of communication and understanding in full before rendering judgment.
I now return you to the rest of the rambling as originally written.
While most people state upfront that it is absolutely essential for a sub to trust a dom, it is also abolutely essential for a dom to trust a sub. Without that trust (which I no longer had for the girl above) a dom can't really push a sub safely. How will he know if she's having a problem if he can't trust her to speak up.
In addition the dom needs to be able to trust that the sub wil not change thier mind about participating in the relationship. Even though both parties are fully involved in the consental nature of the realtionship many of the activities which are done will to the outside obsever, look non-consentual. Should the relationship end on a bad note (as relationships are wont to do) the sub can do irreperable harm to the dom by filing for criminal charges, and there is no protection the dom can use, because even a signed and witnessed contract between the dom and sub covering specifics of what will transpire between them does no more than prove a covenent or conspiracy to break the law. Every time a dom spanks a sub, they are opening themselves to assault charges, and that says nothing about paddlings, whippings, or fantasy rapes.
None of that even begins to adress the trust that a sub woudl be able to carry out instructions unsupervised, and to remember all orders that have been given, as well as rules. It's not the job of a dom to micromanage the life of a sub, though many do so, they do so out of thier own choice.
Now, lest you think I don't consider it important a sub must have the utmost trust for their dom. They are in a very vulnerable position emotionally (and often physically). The very nature of submission demands incredible trust.