Before we jump into how the code might have been wrong, there are other issues that can impact mission-critical applications like this that have little to do with the high-level code. In financial applications it's somewhat common to build the hardware to tolerate SEUs. For example, I know of one application where two identical digital subsystems are used, given the exact same input, and the outputs are compared. If the outputs ever differ a logic error is assumed and operation halted.
Logic errors are rare but not that rare. At sea level you can expect ~1 bit flip in 4GB of memory every 24 hours. Often it's in an invalid line or gets overwritten anyway, but for applications like this where one logic error can cost you your shirt, application of hardware-level error checking (ECC, SEU detection and correction, etc).
Logic errors are rare but not that rare. At sea level you can expect ~1 bit flip in 4GB of memory every 24 hours. Often it's in an invalid line or gets overwritten anyway, but for applications like this where one logic error can cost you your shirt, application of hardware-level error checking (ECC, SEU detection and correction, etc).