Arithmetic Overflow errors coming out of the blue

I have an update statement that has been running daily for nearly 2 years now, which yesterday started failing with arithmetic overflow. These are NOT data related. The query is of the form: UPDATE src SET src.field1 = isnull(t1.key, -1), src.field2 = isnull(t2.key, -1), src.field3 = isnull(t3.key, -1) ... src.field7 = isnull(t7.key, -1) FROM myTable src left join table1 t1 on src.t1Key = t1.key ... left join table7 t7 on src.t7Key = t7.key This fails with an Arithmetic Overflow error. The thing is, if I comment out ONE of the SET statements (i.e. any one of them, it doesn't matter) the UPDATE