If in the middle of the reception of a frame, a constant stream of not in tables and disparity errors are seen, then GMII_RX_DV goes low and the core loses synchronization.
However, GMII_RX_ER is not asserted for the frame.
This sequence happens when using the LogiCORE IP Ethernet 1000BASE-X PCS/PMA or SGMII v11.1 and earlier cores.
Per the 802.3 1000BASE-X receive state machines, GMII_RX_ER should be asserted, but the MAC layer still marks the frame as bad because there is a Frame Check Sequence (FCS) error.
This is scheduled to be resolved in the next version of the core.