Bug In UPDATE Statement And 1 : N Cardinality ?

The problem is in one-to-many cardinality and UPDATE statement :
( SQL server 2005 developer edition )

create table #table1( id int, firstCol int)
insert #table1 values ( 1,0)
insert #table1 values ( 2,0)
go

create table #table2( id int, secondCol int)
insert #table2 values ( 1,10)
insert #table2 values ( 1,20)
insert #table2 values ( 1,30)
insert #table2 values ( 3,100)
go

-----------------------------------------------------------------------------

1. shape UPDATE

update #table1 set firstCol = firstCol +( select secondCol from #table2
where #table1.id = #table2.id )

return error message :

Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
The statement has been terminated.

2. shape UPDATE

update r set firstCol = firstCol + s.secondCol
from #table1 r
join ( select id,secondCol from #table2 ) s
on r.id = s.id

return message : (1 row(s) affected)
and #table1 is updated only one of row ( generally random ) from #table2 for ID = 1

select * from #table1

id firstCol
----------- -----------
1 10
2 0

3. shape UPDATE

update #table1 set firstCol = firstCol + secondCol
from #table2 where #table1.id = #table2.id

return message : (1 row(s) affected)
and #table1 is updated only one of row ( generally random ) from #table2 for ID = 1

-----------------------------------------------------------------------------

Is it normal ? I expected error message or summary value for ID = 1 from #table2 .