That error happens when the inbound flight cannot be accurately determined based on the information we have.
Now the website does not show any inbound flight for 12/30/2105: https://flightaware.com/live/flight/DAL802/history/20151230/2045Z/KATL/KLGA
If the website did show it on that day, it could have been a bad guess.
Flightxml has a more accurate, but more expensive algorithm than the website.
If you find other InboundFlight calls that do not return what you would expect, do let us know and please include the exact timestamp when you notice it.