`Story: I have a strange error when I try to save something I got this error message An entity object cannot be referenced by multiple instances of IEntityChangeTracker. I really don’t know what that is and why is it appear, it appears only when I try to save something my insert and update is working, only when I try to save something in db from my Telerik grid if (this.annualVacationList != null) { List<AnnualVacation> vacationToSave = this.annualVacationList; IEnumerable<AnnualVacation> existing = paramUser.AnnualVacations; foreach (AnnualVacation toSave in vacationToSave) { AnnualVacation existingItem = existing.Where(x => x.AnnualVacationId == toSave.AnnualVacationId).SingleOrDefault(); if (existingItem == null) { ctx.AddToAnnualVacations(toSave); } else { existingItem.FromDate = toSave.FromDate; existingItem.ToDate = toSave.ToDate; existingItem.WorkingTime = toSave.WorkingTime; existingItem.VacationDays = toSave.VacationDays; } } } ctx.SaveChanges(); } After debugging I have seen that the code brake down in the Project.Name.Designer.cs ..... O.o public void AddToAnnualVacations(AnnualVacation annualVacation) { base.AddObject("AnnualVacations", annualVacation); }