Rabban Shimon ben Gamliel, in the final mishna of Ta'anit (4:8), speaks of women going into the vineyards on Yom Kippur and dancing for the unmarried men who would then choose wives for themselves. The women would borrow clothes for the occasion and, in order that they not embarrass anybody (Meiri), all of those clothes would require immersion. To my mind, this mishna is incongruous to the extreme.
Beitza 5:2 expressly forbids dancing on a shabbat. While I can deal with the prohibition of immersing clothes on a shabbat (Beitza 2:2, and cf: Shabbat 22:4) by suggesting that they washed them the day before, and while I can deal with the possibility that the women were all barefoot (Yoma 8:1), I know of no way to explain the fact that they were clearly dancing.
Am I to assume that חולות ("dancing", Ta'anit 4:8) and מרקדין ("dancing"; Beitza 5:2) are two completely different things, such that the former is permissible and the latter prohibited? Am I to assume that Rabban Shimon ben Gamliel is choleq on the tana qama in Beitza? Or am I to assume that, contrary to the plain-sense reading of this mishna, the dancing in vineyards was done on 15th Av only and not actually on Yom Kippur?