We had one customer who had built multiple additions onto their home over the years at various times, and nothing lined up. At one point, they'd had a mural artist paint murals all over their walls, too. All the floors were marble, so we didn't want to install acoustic glass break detectors. Part of their house had a basement and part of their house was on a slab. Lots of big picture windows everywhere. A nightmare install, in other words.
They wanted an alarm and cameras. And an intercom in every bedroom. And a phone system. And a PBX. And ethernet jacks everywhere (this is before reliable WiFi). And CATV. Oh, and why don't you throw in some access control.
It took us five weeks to finish the installation, running wires everywhere without damaging any of the murals. In a few places, we carefully pulled off the (custom, expensive) crown moulding, channeled the drywall behind, used that space to run wires in, and caaaarefully glued the moulding back on.
It wasn't even that big a house, 5,000 square feet maybe. But from a technical standpoint, it was the most challenging installation I have ever done.
Wish I had pictures.