I never read about a priori knowledge in the time of Descartes. The conceptuation of a priori comes from Kant, not Descartes.
I think that there doesn't exist any a priori knowledge, every knowledge is constructed. Sometimes some synthetic judgements seem to be a priori but history shows that all of these judgements are, after all, analytic because any formal science is analytic.
In other way the concept of "knowledge" may requires conscience. The best that you can have about "a priori knowledge" are unconscious reactions as reflex, pain and so on... but this is not knowledge, at least I don't consider it.
But in any way there doesn't exist any fallacy on the thoughts of Descartes as someone said before, because circular thinking is legit and valid. This "fallacy" doesn't exist in reality, history proved it a lot of times.
You can have a process where it doesn't care that it is circular, and it doesn't care what happens first: one time the process becomes circular it exists because it is an equilibrium, and there is no "fallacy" in the real world of this processes. In any case what happens is that there exists some level of unknown mechanism.
I recommend about this topic to read, by example, some book of Paul Watzlawick who studied these things in deep.