When you have a tree heirarcy of objects, and the same value exists in two places of the tree (but with different child values), and when mapping the second instance of the item - it uses the child values of the first instance, instead of re-evaluating what the child values should be.
 
Here is my example:
class Tag {
int Id {get; set;}
string Name {get; set;}
IEnumerable<Tag> ChildTags {get; set;}
}
 
public void Test()
{
var source = new List<Tag>

        {
            new Tag { Id = 1, Name = "Tag 1", ChildTags = new List<Tag>
                        {
                            new Tag { Id = 2, Name = "Tag 2", ChildTags = new List<Tag> 
                                        {
                                            new Tag {Id = 3, Name = "Tag 3"},
                                            new Tag {Id = 4, Name = "Tag 4"}
                                        }
                                }
                        }
                },
            new Tag { Id = 1, Name = "Tag 1" },
            new Tag {
                    Id = 3, Name = "Tag 3", ChildTags = new List<Tag>
                        {
                            new Tag {Id = 4, Name = "Tag 4"}
                        }
                }
        };

 
Mapper.CreateMap<Tag, Tag>()

.ForMember(dest => dest.ChildTags,
    opt => opt.MapFrom(src => src.ChildTags));

var result = Mapper.Map<IList<Tag>, IList<Tag>>(tags);
}
 
 
In the result

• the first instance of Tag 1 (ie source[0]) and all of its children are perfect
• the second instance of Tag 1 (ie source[1]) has all the children of the first instance - it should not have any children
• the second instance of Tag 3 (ie source[2]) does not have any children - it should have Tag 4 as a child