We had someone drop out of our program about 8 years ago to do anesthesia. I have heard of this happening at several other programs as well. I don't think it's a reflection of the oral surgery program, they just changed their mind. The funny thing is that they end up finishing anesthesia a year later than they would have finished oral surgery.
We also had someone drop out of our program for orthodontics after finishing med school about 5-6 years ago. JediWendell has given his thoughts on this before. I guess it's better to find out sooner rather than later that you would be happier somewhere else.
As for as other medical specialties...the more I'm around them, the more I confirm to myself that I chose the best kept secret in all of healthcare, at least for me.