In the second season of Game of Thrones, when
Arya was Arry & Jaqen H'gar was wearing Lannister soldier clothes with two others in a makeshift prison, on their way to The Wall. The cage catches fire & the 3 men are about to die, but Arya, unexpectedly, saves them from death. Jaqen reveals that he is a faceless men & 3 deaths are owed to the God of Death. So he gives Arya 3 people to choose whom he will kill.
In S05E10, Mother's Mercy, when
Arya kills Meryn Trant, she has taken a life which was not supposed to be taken. Hence Jaqen says to her that a debt is owed to the God of Death. He takes the poison & dies, apparently to pay that debt. But how is that supposed to pay for the debt. His death was also unexpected. Doesn't that make the debt count of God of Death to 2 rather than 0.
TL;DR; In earlier season episode, 3 lives were saved, a debt was repaid when 3 guys are later killed. In S05E10, an unexpected life was taken, which was repaid for by another unexpected death. So how is the debt settled?