I had that with my current car, I've only had it 9 months now, but when it came time for a new tyre they went to get it off, problem, the locking wheel nuts wont come off - then - the adaptor part has now broken, of course they had been done up with a gun - my tyre place is good, they always put them on by hand and then use a torque wrench to tighten them. fortunately in the packet that had the locking 'key part (both were broken) there were the old studs, I had to wait all day for the car instead of just an hour while the tyres were done, as they had another car in front of mine to have its locking nuts removed too, but it was done and my new tyres put on - for just an extra £20 onto the cost of the tyres, have I bothered to put new locking nuts on - no I jolly well haven't, as I don't want to be stuck with no way to change the tyre.
Ps I always have a 24" breaker bar in the car for wheel nuts - just in case
Grendel