I have over 60 trips thru the airport and I have seen it happen only once and this was about five years ago. Its was a young kid, long hair, backpack only, one of the customs official came up to him and inquired if he had the funds to cover his stay.
The only other time was concerning a round trip ticket purchase. The guy beside us in Miami was raising heck about having to purchase a round trip ticket to Belize. Showed the agent some paperwork and eventually must have been sucessful. However when we landed in Belize here came an airline representative and an immigration official. He had to buy a return ticket to Miami before they would allow him entry.
So it does happen.