The single most inexplicable and corruptive tenet of Catholicism through the ages.
Indulgences: a stumbling block for the Protestants and a foolishness for the Orthodox ;>
A Scriptural "stumbling block," as is "Purgatory."
Over the ages the CC has amassed an untold fortune convincing untold millions -- rich and poor -- that monetary "Indulgences" would influence and limit time spent (in leiu of penance) in "Purgatory."