I have seen that a lot in the garage business. Also had a Ford with a drivability problem that had just came from a quicky tune franchise. They hadn't gapped any of the plugs and it was setting a pip (profile ignition pickup) code.
I dropped a spark plug doing a V8 Monza tune up, ironically the front left one that hadn't been changed before. After the tune it had a rough running issue. I finally found it on the scope. I couldn't see the crack but it must have cracked the insulator on that plug when I dropped it. I have never put a dropped plug in a car since then and won't accept a box that show any sign of having been dropped.
There's lots of ways to screw up a plug swap. including sloppy work. Unacceptable. If that's how he does his work I would send him down the road.