Abandoned-in-place wiring is the bane of every tech's and installer's existence. It's impossible to have worked on legacy equipment and not spent some time reterminating abandoned cable :/ Not to mention having to weed through it when tracing problems.

We're in an ongoing battle with abandoned-in-place wiring in our current building. There's data wiring back to ARCnet (93 ohm coax), and power wiring going back to knob and tube days.