Tao,

I think the difference is just in the wording of the error message. My SQL 2000 instance gives: Msg 245, Level 16, State 1, Line 1

Syntax error converting the varchar value 'B' to a column of data type int.

and my 2005 gives:Msg 245, Level 16, State 1, Line 1

Conversion failed when converting the varchar value 'B' to data type int.

Note that both versions call it Msg 245, implying that it's the same error and the text has been corrected/clarified in the newer version. It's not really a syntax error at all, but a conversion error at the time the UNION is attempted.