A user opens the app, sees a zero balance, and you are not debugging anymore: you are managing trust.
The Crypto Times reports that Unocoin, India's oldest crypto exchange, froze logins as 81,000 wallets showed $0, under a headline that still asks whether it was hacked. The cause is not stated there. That gap is the one worth closing in your own system: where hot-wallet keys live, what a withdrawal has to satisfy before it is authorized, and which servers can reach either of those.
I review those three boundaries for exchanges and wallet operators. A focused review is measured in days, and you get scope and timeline within a day. If you hold other people's funds, send me your withdrawal path and I'll tell you where I would look first.