What is "The Miracle" that 10,000 fans of KimiUso voted for? 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.
http://live.nicovideo.jp/watch/lv218533566

We can not watch this live streaming already, but we can read the viewer's blog.
I reprint and translate the important point.

Japanese


  宣伝Aさんいわく、「何も考えてなかった」と。


English


  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