One of those which comes up in the book is the practice of forcing mothers to abandon twins, called osu, because they represent evil. The Ibo believed that they needed to be sure and distance themselves from these evil beings and so, in Umuofia, they had to abandon the babies in the evil forest and leave them there to die. The missionaries were horrified by this practice and ended up saving the babies and this was one bone of contention in the story.
The missionaries also felt that polygamy was an abomination and that it was not appropriate. This led to further conflict with the tribe as it was obviously an integral part of Ibo culture.