A method for creating distributed public key for ECDSA, what are the risk factors?
Suppose Bob watches Alice publish $A = [a]G$.  Bob then picks $b$ and publishes $B = [b]G - A$ as his share of the public key.  Now the combined public key is $A + B = A + [b]G - A = [b]G$, so Bob can unilaterally sign messages without Alice's consent.