It sounds like maybe this was somehow a misunderstanding, which does sometimes happen, perhaps she just for whatever reason remembered it differently and t hought you said you would call if you weren't able to go, or whatever. Those things happen from time to time, and the other person could have sworn you said such and such and you know you didn't. FOr those type of things, I just try and allow for that possibility, that she misunderstood and thought yo uwould be going along, so I would just call her up and even though you didnt do anything wrong, kindly apologize and say that you're sorry for the misunderstanding, and you had meant to convey you were not sure if you'd be attending, but want her to know it was thoughtful to get the tickets nad thank you type of thing.