The Basel II banking regulations aimed for a 99.9% probability that banks could cover unexpected losses. However, due to the way capital requirements were structured, banks may have actually only had a 95%-99% chance of solvency. This means that an average-quality bank could face failure once every twenty years under these rules, even when considering their interest income. The risk of failure increases with the riskiness of the bank's loan portfolio. This suggests that Basel II may have underestimated the true risk of bank failures during the global financial crisis.