The sinking process is both intense and delicate. As the drum is sunk, the pan maker must ensure that the metal is not overstretched. If this happens there is a possibility that the metal will split.
If a split occurs some pan makers will discard the drum, others will weld the split and continue with the pan making process.
Although many of the welded pans can be tuned to an acceptable level, many people avoid them as the welding process affects the overall tone of the pan. If the weld is across a note, the tone of that note will be affected and may cause problems for the pan tuner when tuning.