There was this drastic ideological shift for me which completely exploded my perspectives.
I remember this peculiar instance while working on a project from Diebold (which makes ATM machines). Coming from a mech engineering perspective, when I looked at any product/object I could just see through them and how they were manufactured or at least imagined how they were. There was just no other way in which those products could be made.
In this particular case, I was redesigning the entire ATM experience, right from the vestibule, to the position and placement of the machine, the advertisements and kiosks for forms and slips, the ATM machine itself and the screens on it. It was a huge project. I ended up constricting myself and making a box shaped ATM more like the current ones barring certain changes in the screen size, position of the cash outlet, card inlet, etc.
This was because I knew how money is loaded and dispensed and replenished, the entire working of the machine. I proposed a vestibule layout change along with minor changes in the machine mentioned above and revamped the entire interface. The turning point was when I was asked why is the machine of the future so big? I had the mechanical answer in my head. While product design philosophy insisted that it needs to be user friendly and not necessarily manufacturing friendly at the moment. Technology will rise to it. In fact, the entire cash dispensing mechanism could even be put underground! Just a outlet for the cash to come out! This thought changed my ideology completely. I understood how engineering/technology should always be the facilitator and how design should always spearhead product development. If it were left to engineering, we would be stuck in the iron age!
Sadly we still live predominantly in an industrial world where design has a long way to go. User centric design is far off and presently the developer-centric and manufacturing-centric design will keep its head high.