According to the wikia, fans had a chance to vote on if they wanted a miracle to occur. This was during the run of the series. What is this miracle? Information in Japanese can be found on the second link but it is not easily Google Translateable.
There was the KimiUso Staff Talk Event at 2015/05/16 in Tokyo Japan.
"The Miracle” was explained by the staffs of KimiUso in this event.
We can not watch this live streaming already, but we can read the viewer's blog.
I reprint and translate the important point.
PR staff A told us to "I was not thinking about anything".
As it turns out, there was no miracle.
This is the most official conclusion.
We Japanese fans are thinking that the Miracle was the staff A's Uso = Lie :p