As "Not telling" mentioned in a previous post:
It got me thinking. What is the worst repairs that you've seen? What was done, and with what was that done?
This may well bring up some interesting cases.
I'll start, although this may not even come close to what others have seen: I remember a violin on eBay, where the cracks were repaired with white glue. The white glue literally peeled out of the crack. And it was shown on the photos.