As an actual gift, not something that is not normally a gift (Like Madyyyy's elephant poop), the worst thing I could receive would be the thing I had given someone else for THEIR birthday. As in a re-gift from the person, with the exact same item, not a similar item.
A Bible and jesus on a cross...and you're not even christian. A math textbook, deodorant followed by a grin and thumbs up by the person who gave you it, your grandparent dying, TWILIGHT SERIES D': but even I'm not that mean.
A demand from the President of the U.S. saying I must pay him 2 million dollars before 1 month's time or else I would have to be shipped in a crate to Guatemala and be forced to be a street performer for 2 years.