Arduino power supply

Is it possible to power up Arduino with DC power adapter, and with USB at the same time, so it is always powered by DC 12V and whenever I want to reprogram it I can put USB in. Would there be something wrong or not?
Thank you

Not only is there nothing wrong, but I think it is pretty normal practice, although I submit 12v is not a good idea. Life is more comfortable with a 9v supply.

Do it all the time, with robot kits…

My UNO derivative that you can see HERE: has a switch-mode power regulator so 8 to 20V is fine, and it provides up to 2 amps at 5V.

DISCLAIMER: Mentioned stuff from my own shop…