Curiosity found, the wrap
After a good nights sleep when almost all pieces fit together here
weblogs.sqlteam.com/peterl/archive/2008/02/06/Curiosity-found.aspx
I realized this morning that this behaviour also explains why there are gaps in identity sequences when inserting a record that violates a contraint.
It seems that identity values are assigned to complete insert-set before checking for constraints such us unique indexes.
DECLARE @Items TABLE (i INT IDENTITY(1, 1), j INT PRIMARY KEY)
INSERT @Items
SELECT 1 UNION ALL
SELECT 2
SELECT *
FROM @Items
INSERT @Items
SELECT 1
INSERT @Items
SELECT 2
INSERT @Items
SELECT 3
SELECT *
FROM @Items
INSERT @Items
SELECT 1 UNION ALL
SELECT 2
SELECT *
FROM @Items
INSERT @Items
SELECT 1
INSERT @Items
SELECT 2
INSERT @Items
SELECT 3
SELECT *
FROM @Items