User has ₦100 in their wallet. They open two tabs and click 'Withdraw ₦60' in each tab at the same time. Both requests read balance = ₦100. Both pass the sufficient-funds check (100 >= 60). Both post withdrawal entries. The user now has ₦100 - ₦60 - ₦60 = -₦20 in their wallet, and ₦120 of withdrawals succeeded against ₦100 of actual balance. They double-spent ₦20 of money that doesn't exist. This bug is the most common production fintech outage. Three fixes exist: pessimistic locks (SELECT FOR UPDATE), optimistic concurrency (version column), and database CHECK constraints. All three solve it; engineers building serious ledgers use at least two.