Wow! I did not expect so many responses! General opinion seems to be the travel violation. As a new ref, I don't find it much difficult to call... at least not yet! I probably don't know the rule enough... However, I admit I sometimes anticipate the call and not the play, so I call too fast and realize a split second later that I am wrong...
Fortunately, I am aquiring more experience for each and every minute I spend on the court.