In a confidential transaction, what guarantees that the sender packed the correct ephemeral public key for the pederson blinding factor?
What guarantees that the sender packed the right ephemeral public key? What's stopping the sender from packing a different ephemeral public key, preventing the receiver from figuring out the blinding factor and the transaction amount. 


The fact that the receiver wouldn't treat the result as a valid payment.

This is no different from them sending the right amount to the wrong address.