Many years ago, I've noticed that one without requirements of any 5V devices like USBs, he/she can completely bypass the power input circuit and the regulator. Feeding Pi Model A with power from v3.3 GPIO pin, and shorts 5v with 3.3v can be a practical solution.
I knew it should be proceed with caution since there was no protection, e.g. feed 5v to 3.3v could destroy it. I also found some nice measurements for it.
However, time passes and it's 2016 now. I do not know if the circuitry within Pi 2 or Pi 3 has been changed and made this approach insane. I wonder if it could fry some parts like the regulator.
Can I still power newer Pis (2/3) from 3.3v without any issues?