O indice esta invertido, ele deve ser acessado do maior para o menor(i--).

https://github.com/DanielleMoura31859798/Source-Code-Inspection/blob/56b0f5f298c765bf108a41a45cc744f46a5bd433/Source%20Code%20Inspection/src/br/calebe/ticketmachine/core/Troco.java#L72