Todd H. wrote:

What are the top 10 things that folks have actually seen (or heard
directly from someone who saw, no hypotheticals please) screwed up by
prior installers, and the potential consequences of those screwups?

I don't know how this ranks, but I had to fix a friend's furnace when she bought
a couple of years ago:

The installer had put the filter holder on the side of the furnace opposite the
ducts. The filter was right up against the wall of the furnace. I had to
remove the filter track and re-install it on the side where the ducts were.
There were a bunch of little bits to tidy up in the process.

My guess was that it was installed by someone who'd never seen a furnace before.
Since the house was only a few years old, it was the original property
developer's team of "expert builders". The warranty had expired a couple of
years earlier, otherwise I would have made an amusing phone call on my friend's
behalf.

Mike