Not 100% sure how to turn it on/off in Entity Framework but basically what it's telling you is that between when you grabbed the data out of the database and when you saved your changes someone else has changed the data (Which meant when you went to save it 0 rows actually got updated). In SQL terms, their update query's where clause contains the original value of every field in the row, and if 0 rows are affected it knows something's gone wrong.