There were instances when a niggun was officially chosen by the Va'ad but another niggun turned out to be the niggun of that year. One such instance was in 5749.
Reb Sholom Halevi Bruchstat relates:
“The Va'ad had chosen a niggun as in the past two years; however, there was a significant Olam who disagreed with the choice made. They sang another composition and this created quite a bit of confusion. In order to make a seder, on Shabbos Hagadol, it was announced in 770 before the farbrengen that everyone should sing the niggun that the Vaad had chosen.
After the Rebbe entered the farbrengen and made Kiddush, Reb Moshe Teleshevsky led the crowd in the niggun that had been chosen by the Va'ad (beginning with the words “Tavoi Lefanecha”). To the surprise of Chassidim, the Rebbe immediately began the first sicha (all but ignoring the niggun). The sicha explained the opening posuk of that year’s kapitel “Shir Mizmor” The only niggun including these words was the one that I had composed.
Naturally, the niggun that was sung immediately after the first sicha was “Shir Mizmor” and that became the niggun of the year.
Many wondered how I knew to include that posuk in the niggun. The truth is that I remembered that the Rebbe had spoken about that posuk the year before and I figured the Rebbe would speak about it again this year.
When the Rebbe spoke about that posuk and the niggun was accepted I was elated! I felt that Rebbe had “come to my defense” so to speak.
(After the farbrengen, Reb Sholom took a lot of Mashkeh and danced for hours, celebrating that the Rebbe had chosen his niggun. To further appreciate his joy, it should be pointed out that his niggun had not been chosen for the previous two years.)”
Reb Sholom Brochshtat
From the album "Music Notes"