If you have a column or table name that is a reserved word or which contains spaces etc. you can force NHibernate to use quoted identifiers by using a back tick in the table/column name:
<class name="LineItem" table="`Line Item`"> <id name="Id" column="`Item Id`"/><generator class="assigned"/></id> <property name="ItemNumber" column="`Item #`"/> ... </class>