In Pay On Delivery (POD) mode there may be many reasons for failed delivery/transaction of Goods/money like
(1) the address may be wrong/ untraceable
(2) the recipient may not be available etc
(3) the customer can refuse to pay & accept the goods delivered etc
under...
more... these circumstance the goods is transported back to seller and then sold to another customer,
But in the case of Reserved tickets which is a perishable commodity i.e. is useless once the day of travel is over. Failed POD may prove to be a big headace to IRCTC