I had my clutch replaced in the not too recent past by a shop. As soon as I started it the car shook quite a bit with even moderate engine RPM. I shut it off, walked back into the store and told them to re-do it. They um'd and ah'd but I insisted, telling them about the only way a new clutch could vibrate this bad is if it was installed without dowels installed in the flywheel, certainly possible if the flywheel was resurfaced and the dowels were removed but not put back in afterwards.
When I got it back, they acknowledged the missing dowels and the car was smooth as butter again.