Budvar10:
Definitely it is not a problem. If the board is UNO or MEGA, the most used, there is on-board switch between USB and jack/VIN. Jack/VIN has priority (VIN > 6.6V). You can connect both (VIN and USB or jack and USB) at the same time.
That is true, but only when talking about original Arduinos/Genuinos.
So called "compatible" boards or clones may have different schematics.
Connecting USB plus Vin with these boards may produce some smoke 