Box of chocolates?
To paraphrase Forest Gump, classic car restoration is like a box of chocolates. You can never be too sure of what you're dealing with until you start peeling back the layers.
We noticed some 'cracks' in the rear quarter when we first brought the Mustang back to the shop. As we dug further we found the rear quarter panel had been entirely cut off and replaced at one time following a collision. Under the plastic body filler you can see the seams where the shop brazed another one on (notice the brass filler). It dates back a ways since brazing isn't really used anymore. The technique predates MIG welding which is common now.
There's evidence that it was wrecked again in the same place, but the second time they drilled holes and used a slide hammer to pull the dents out best they could. The holes weren't welded shut and instead, plastic body filler oozes through on the back side like Play Dough worms. The problem with this is, moisture can then enter from the back side and start rust and paint bubbling.
The Michelangelo that made the second repair spent a lot of time and did a decent job of sculpting body lines with body filler but you can see where this is going.... We're going to cut it all off and put an aftermarket quarter panel back on. It'll require some surgical skill and patience but should result in a sound repair in (hopefully) less time.
The door had also been replaced (it's a different color). After which wreck, we don't know. The driver side door has also been repaired along the way and the rear valance is toast.
Below is a video with details...