Mr. Obama was boarding the presidential helicopter, Marine One, when he "misjudged the doorway and banged his head on the low doorway," as the Associated Press reports. The photographic evidence is above.
The incident occurred when Mr. Obama was getting aboard the helicopter for a trip to Maryland's Andrews Air Force Base, where he boarded Air Force One en route to a town hall meeting in Indiana.
As the AP notes, this morning's trip was the president's fifth on Marine One. One imagines that next time he gets aboard he'll remember to duck.