A "design anomaly" in the way Microsoft Windows servers were integrated into a Californian air traffic control system left 800 planes in the air without contact to air traffic control, it has been revealed. A combination of good old human error and a design glitch in in the Windows servers led to at least five cases where planes came too close to one another. Air traffic controllers were reduced to using personal mobile phones to pass on warnings to controllers at other facilities. The Windows servers were brought into Southern California's air traffic control system over a period spanning the last three years, in order that they replace the radio system's original Unix servers. The Windows servers were running Microsoft Windows 2000 Advanced Server and using Dell hardware. Microsoft have not commented on the issue.