A Pet Peeve...

Extraordinary ministers of Holy Communion giving blessings as if they were priests.

This article gives various reasons why it's a bad idea, among them:

A 1997 instruction, Ecclesia de Mysterio, on the collaboration of the lay faithful further indicates that the laity should never say prayers or perform actions during the Mass which are proper to the priest, as this may lead to a confusion of roles. Since the blessing of the congregation during Mass is reserved to the priest, lay persons must avoid doing so.

If you attend a Traditional Latin Mass, of course, this problem is avoided altogether...