GR vs QM

[Dobin]

What i think i know:

So GR is perfect for describing objects with extremely large masses, such as celestial bodies, and QM is what we use to describe atomic and subatomic phenomena.

But we've all been told they don't cooperate very well when used together, with cases such as black holes, which have extremely large mass, but are extremely small. Hence the search for a unifying theory, such as string theory.

What i would like to know is what exactly happens for these two to contradict or disagree etc? What happens with the equations or predictions the two theories make?

[Little Bang]

Gravity