Possible when both terminals of the dud and donor are directly connected and the dud has an internal fault. Negative (for most, but not all cars) should be connected to a decent bit of metalwork on the dud vehicle -NOT the negative terminal of the dud battery. Unfortunately many people do things the lazy/wrong way. A competent mechanic should know better. The correct method is described in most manufacturer's vehicle handbooks. The incident you describe won't have done the donor battery any favours either - best to have it checked.