Holding hands during the Our Father.
This is oddly widespread in the United States but it’s an illicit addition to the liturgy. The official publication of the Sacred Congregation for the Sacrament sand Divine Worship, Notitiae (11  226), states the practice “must be repudiated . . . it is a liturgical gesture introduced spontaneously but on a personal initiative; it is not in the rubrics.” And anything not in the rubrics is unlawful, again because “no other person . . . may add . . . anything [to] the liturgy on his own authority” (ibid).
Notitiae (17  186)) also reaffirms that the priest may never invite the congregation to stand around the altar and hold hands during the Consecration. He stays in the sanctuary and we stay outside of it.