[danharaj]

The equation that defines curvature in GR depends on the stress-energy tensor which describes the distribution of matter/energy in spacetime. This depends exactly on position and momentum of the matter involved, which is in direct contradiction with the quantum mechanical nature of the stuff.

Normally this doesn't matter because gravity is so weak compared to the scale where QM effects dominate but it is a mathematical inconsistency that becomes very relevant towards the extremes of both theories, in particular: the very early universe just after the big bang and the dynamics near black holes.