From what I see, the complexity of these units is just incredible, the amount of wires, components, connectors etc. It would be very easy for a mistake to be made. Human Nature.
My main weakness with re-assembling tape decks would be leaving the counter belt off, I did this 5 TIMES! and had to pull the whole mech out of most of them to put it back on.
With the nakamichi 480 I blew the crap out of the recording section when trying to replace faulty components on the board (my soldering sucks), it literally smoked. I'm thankful its still a great playback only deck