What we hope happens

sequenceDiagram Withdraw --> Balance: Note right of Balance: balance=100 Balance ->> Deposit: 100 Note right of Deposit: Dep = 100+100 = 200 Deposit ->> Balance: 200 Note right of Balance: balance=200 Balance ->> Withdraw: 200 Note left of Withdraw: Wit = 200-100 = 100 Withdraw ->> Balance: 100 Note right of Balance: balance=200

What might very well happen instead

sequenceDiagram Withdraw --> Balance: Note right of Balance: balance=100 Balance ->> Deposit: balance: 100 Note right of Deposit: Dep = 100+100 = 200 Balance ->> Withdraw: balance: 100 Note right of Withdraw: Wit = 100-100 = 0 Withdraw ->> Balance: set: 0 Note right of Balance: balance=0 Deposit ->> Balance: 200 Note right of Balance: balance=200