On Feb 24, 2015, I used my Firstbank Mastercard for multiple withdrawal transactions on an Ecobank ATM machine located at Aero office in Port Harcourt. Mid the transactions, a requested sum of twenty thousand naira failed to dispense and my account was debited. I have long contacted my bank via several allowable means and I get response that Ecobank insists that the cash dispensed.

Ecobank sent an un-verified transcript to FBN stating that all six transactions I did dispensed and my bank, First Bank has remained clueless and helpless on retrieving my funds from the failed transactions.

I wonder how many people that get same experience and suspect systematic gaps in the ATM program supporting this type of fraud.

Nairalanders, please what do I do?

I have experienced it before but the system later reversed itself. In another bank where I experienced such, they had to print out their daily journal from the ATM and reconciled it manually. My fund was later refunded also. But the two instances were not inter-bank transactions. I believe this made it easier.
As my own personal policy, I don't use my ATM in another bank's ATM. If you practise this, it will save you a lot of headache.