Hasidim perform hakafot on Shmini Atzeres [Translation: The decendants of Hungarian and Polish peasants who present themselves today as the epitomy of religious Judaism perform the ritual circling of the bima on both Simchas Torah (ie: the Ninth Day) and Shmini Atzeres(ie: the Eighth Day)]
Why is this absurd?
Refer to the Rama where he writes that the hakafa custom (together with other customs, including the in-shul feasting, the aliyot, kol nearim, the singing, the dancing, the reciting of praises, etc.) was created for the express purpose of celebrating the completion of the Torah. This occurs on the Ninth Day (in the diaspora*) and the Rama, therefore, says nothing about performing the rituals of celebration on the Eighth Day. They belong to the Ninth Day and the Ninth day alone.
So why do Hasidim do hakafot on both days?
My guess is some Rebbela from long ago was unaware that hakafot were created in the diaspora. And because disapora Jews do the Seder twice, he probably thought it would be a good idea to perform the hakafot twice, too.
[*The mechaber, who wrote from the perspective of a Jew living in Israel, says the Torah is completed on the Eighth Day and he makes no mention of the hakafot and other diaspora-created customs. ]
[Update: I see many people have complained about this post, so let me put a fine point on it: There is only one reason to do hakafot: TO CELEBRATE THE COMPLETION OF THE TORAH. That's why the custom was created, by disapora Jews, for the NINTH DAY. Anyone who performs the custom on the EIGHTH DAY in the diaspora is demonstrating ignorance of the custom's origin.]