Some of my personally-experienced gunsmithing foul-ups; all from very popular and "reputable" shops:
-A Remington BDL stock I send to be re-finished that came back with a 6" crack they swore they didn't do.
-A CZ 452 (not the one I currently have and have posted pics of) that I had threaded 1/2 x 28 and the threads were way too long.
-A Browning Buckmark bull barrel that I had threaded and they were supposed to move the front sight back...front sight installed a few degrees off.
-Very recently an AAC 51-T brake pinned/welded by a local precision rifle shop. Mount threads damaged to the point that the suppressor canted upon lockup causing endcap strikes.