DbUpdateConcurrencyException on Unchanged entity

It appears that I am getting a concurrency exception on entities that are not modified. I am using Entity Framework 4.3.1, Code First, and this is happening in a WCF service.

To simplify my example, let's assume I have two entities.

public class Foo
{
    public int Id { get; set; }
    public string BarKey { get; set; }
    public string SomeValue { get; set; }

    public Bar MyBar { get; set; }
}

public class Bar
{    
    public string Key { get; set; }
    public string OtherValue { get; set; }

    public ICollection<Foo> Foos { get; set; }
}

The user can only edit Foos through the application. Bars can be viewed, but not edited. Additionally, I use DTOs with the WCF service. The typical applicaiton flow is as follows.

1. Client calls service method GetFoo. This will retrieve Foo from the database and map it to a FooDto. This will also retrieve the related Bar and map it to a BarDto.
2. Client updates FooDto
3. Client calls SaveFoo and passes in an updated FooDto.
   a. The service will re-retrieve the original Foo, using context.Set<Foo>().Find(fooDto.Id), then copy the updated values from FooDto.
   b. Business logic runs, which may call context.Set<Bar>().Find(foo.BarKey). Values may be read from Bar, but it will not be updated.
   c. The service then calls context.SaveChanged().

At this point I occassionally get a DbUpdateConcurrencyException. In my error handling, if the exception is of type DbUpdateException, which is the base class of DbUpdateConcurrencyException, I loop through ex.Entries and logentry.Entity.GetType() and entry.State.

In my error log I have several entries that show the type is Bar and the state is Unchanged. It is entirely possible that Bar could have been updated. However, I do not care because Bar is unchanged.

Why am I getting this exception and how to keep it from happening?