Most banks, airlines even the military use legacy software because to replace it costs millions. Instead, as companies grow or change, old software is merged with new software. Aleks hears about ‘technical debt’, when software engineers who create original software code leave or move on, taking their expertise with them. Without proper knowledge of the old code, maintaining legacy software can become problematic and leave a company or organisation vulnerable to technical bugs. The damage brought by thee bugs can leave a legacy of its own. And Aleks asks whether the software is really to blame? Producer: Kate Bissell Researcher: Anna Miles