Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of speed [REF].
Installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging, which requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector.