I don't have the memory capacity to recite them all, but a few that come to mind is installing the incorrect Flywheel Bolts that were too long and damaged the rear Thrust side on the Main Bearing. Another is loading the Differential assy. on the wrong side of the Axle assy. Another big mistake has been to follow the wishes of a Customer in not replacing the Body wood in their vehicle during a restoration. Another that comes to mind is incorrectly repairing a broken Bead on a Fender and not installing a long enough Edge Wire to keep the stresses from re-cracking the Bead.
There are many, many others that will surface given enough time. Generally speaking, Learning comes from Experience. And as they say,
Experience is something you get after you needed it!!