It depends on how you do your "wrong reagent defrosting". If for example you do not completely thaw the reagent and remove some of the liquid then you like have incorrect proportions of the reagents in the portion that is thawed. If you heat up the buffer to high you could potentially denature the polymerase (most polymerase have a half life of 40 minutes at 95C. This means it is still possible to overheat the sample. Likely if you messed up too much with the reagent concentration then your PCR simply would not produce any product.