Interstitial defects in crystals: "high energy configurations"?
Yes, interstitial defects increase potential energy, as stated in Wikipedia: "In interstitial defects three or more atoms may share one lattice site, thereby increasing its total energy." See Physics on StackExchange for an explanation of potential energy in crystals.

How can they form? One example is during rapid crystallization, where there is insufficient time for atoms to segregate. Think of this as "freezing in place" a defect. If warmed and cooled again, the defect can be excluded and a more-perfect crystal forms. This is the basis for the zone refining, Czochralski process and float-zone purification, where extremely slow crystal growth allows time for impurities to be pushed out.

On the other hand, one can intentionally induce interstitial defects through repeated folding, heating, and cooling to make metals harder and tougher, such as Wootz or Damascus steel. It is easy to see how this laborious process requires energy to insert the defects.