I think this is an issue with LWIP. I have described the problem and provided the code at Xilinx Forum. The problem occurs when receiving UDP packet at high TX data rate. The issue result in wrong reference pointer to target packet ID returned by lwip_recv or netconn_recv. Ethernet NIC received the pbuf correctly in memory.
Here's the original post detailing the problem and how to duplicate it. The version is 2.02 (latest in Xilinx port)