Case studies

Problems I found, fixed and proved

Anonymized write-ups from client work. Each one: what I saw, what I decided, and how I proved it.

Card settlement above authorization: negative balances

Purchases that settled above their authorization took card balances below zero, and nothing reported it.

70 to 89 % of over-settlements at tip merchants would fit inside the reserve, by my calculation on replayed real traffic from before the change.