It isn't an error. Someone opened the set and swapped out the quarters. Since 1999 the set holders have not been sealed and can be opened and closed without showing any evidence of doing so. It lead to some interesting
ebay scams back in 1999 and 2000.
The first scam took place in 1999 when some "rare error" sets were "discovered" that had one of the
State Quarters missing and two examples of one of the others. These were going for $300 to $400 each. Of course all you had to do was take five 1999 proof sets at $30 or so each, play musical chairs with the quarters and have five $300 "error" sets.
The next scam took place in 2000 when some "Rare VIP" ten coin 1999 proof sets started showing up on
ebay. The 1999 sets were nine coin sets because they came out before the proof
SBA's were issued. These "VIP" sets had all nine S mint proofs and the P mint proof
SBA. They were going for $1,000 a set or more. What they actually were where 2000 sets (which had ten coins) that had had the coins removed and replaced with 1999 coins and then put into 1999 boxes. You could tell because changed the background color of the proof sets between 1999 and 2000 and these had the 2000 color background.