I spoke with a friend of mine who use to work at the MB dealer in Charleston, SC and he told me the crack was probably caused by improper preparation during the welding process. He said there was probably some sort of contamination. I showed him the photo of the hairline crack and he said the weld bead looked fine and that the weld did not hold because the parts were not free of contamination. He said that crack was probably there for a very long time and that it was bound to fail but that I should not feel bad for not knowing about it because most people don't routinely check for exhaust leaks and residue on the heat shields.