Correctly done, it should not be a problem for the patient. However, a number of factors can lead to complications, and even permanent injury . There can be other factors, but you get the idea. The injury can be serious. Misaligned teeth can result in pain that worsens over time, rather than subsiding.
Inflammation can develop in the joints, making the problem worse.
Inflammation in the joint can move the jaw out of alignment. The temporary crown must fit. Your jaw shifts to compensate for the incorrect fit, and may develop some inflammation of the joints. He does some grinding on the crown to make it fit better, some of the inflammation goes away, your bite shifts back toward where it was before, but now the crown isn't right for that location. The dentist ends up chasing the bite, with no end in sight.