Responsibility on employer for misconduct of employees

If an employer X employs Y to perform duties on his behalf that involve customer interaction, then will X be responsible for any mischief done by Y while on duty. For e.g. if Y misbehaves with cutomers of X or causes mental or financial damage to the customer then how much liability comes on X ? This is assuming the case where Y was properly trained by X and also instructed on his limits while interacting with the customers.