As Bob mentioned, it was a disc thrown by 1983's human player - the last colored disc as far as I can tell. You can see it hit the 2 point goal in
this video from our robot (about 2:12). It was the only disc scored in that goal, and along with the red color on a red background it is understandable that it was missed in the count.
The last two matches were incredibly close, and were very hard fought with 1983's compressor out. We were fortunate that they had enough stored air left to nail their 4-disc autonomous in the 2nd match, after that our alliance's strategy was all about defense, and sneaking full-court shots in over 3789's blocker.
It is unfortunate that the disc was missed and the final round score was overturned after the winners were announced. It was a difficult situation for all those involved, but I commend the volunteers in the field crew, the referees, and the scorekeepers for addressing the situation as thoroughly and as quickly as possible. I also commend 360, 2557, and 3789 for handling the announcement as gracefully as they did.