Do you own a 4-wheel or all-wheel drive vehicle and have had a tire fail? Even if the tires are relatively new, you must replace all four tires. "Why" is very important.
While there are many traction advantages with an AWD system, this is gained by mechanically connecting all four tires. When you introduce a new full tread depth tire on a vehicle with three less than new tires, an overall diameter difference is apparent. When you have different overall diameters, the rotation speed or revolutions per mile will change. This difference in rotation can cause extra heat to be generated potentially resulting in premature failure of driveline components.
Is there another option? Tire Rack does offer a tire shaving service to shave the rubber down to match the exisitng tires using a special machine. While you are certainly reducing the life of the new tire, it is more cosy-effective than risking driveline damage or replacing all four tires.
You can read more here:
Matching Tires on Four-Wheel Drive and All-Wheel Drive Vehicles