If your .dxp file has two related data tables, and those tables have columns which have the same name, but different case (upper vs lower), then when trying to use these two columns in a custom expression, the custom expression tab may fail to recognize the correct table resulting in errors in the expression when the expression editor is closed. For example, the column name may have some or all characters in upper case in one table like "MyColumn", and a differing case in the second table like "mycolumn".