The most common instance of this will occur when an account holder held a short option position prior to the ex-dividend date. In the event a long options holder exercises the position prior to ex-dividend date, the short holder may be assigned. Such action may lead to an account holding a short stock position over the ex-dividend date and will therefore be charged the dividend.
A second instance may involve a client closing a short stock position on the ex-dividend date itself. As the ex-dividend date reflects the first date the security will trade without the dividend, closing a short position on the ex-date will not remove the dividend liability. In order to avoid paying a dividend, the shares must be closed prior to the ex-date.