when product is in more than one category
it gets multible rows in database table: virtuemart_product_categories
when selecting a category to sort products,
ordering numbers for that category are shown ok in BackEnd
but changes might get saved to wrong row for other category
so there has to be a check that "virtuemart_product_id" AND "virtuemart_category_id" is right/match
(or wrong "virtuemart_category_id" are excluded/mismatch)?
[SOLVED] here: http://forum.virtuemart.net/index.php?topic=108844.msg370531#msg370531