Gotta test before you put load on. We've had new transformers that were supposed to put out 120/240 that were putting out 160/320. Actually had one stay on line for a few hours before someone called and it didn't destroy anything luckily. I didn't do it, but I've been guilty of it and got away with it.
Tap changer could have been wrong or maybe wye two primary bushing hooked up to delta or it just could be wrong from the factory.