1) Not to place one’s hands behind one’s back.
2) One should not cut their nails on Thursday,
as they will regrow on Shabbos.
I have seen a choshuveh Rosh Yeshivah walking with his hands clasped behind his back ALL THE TIME.
Hashem makes your nails grow. Presumably if it were an issur, He would stop the growth, Your nails and hair grow ANYWAY, every day, so even if not cut, they will continue to grow on Shabbos. (Did you know that nails and hair even continue to grow after death, for a short while?) I really don’t get this particular issue.