i know since sql 2000 there have been some changes to the way queries are evaluated... you get the same type of errors is you try and insert from a column containing character data into an Int field, even if you have excluded all of the char data and there is only int data stored in a char field.

SQL isn't going to evaluate every peice of data in your source column, so if it's a char(5) then there is a potential for a data clash

try casting the selected column as char(4)