X-Ignore-X is longer than close, which I suppose would mess up the packet length. Or maybe having an unrecognized value for the Connection key would still default to a close? Just guessing here.
TCP checksums are fairly simple; a TCP stack basically just sums up the 16-bit words in a packet and stores the result in the checksum field; this will not detect 16-bit words being swapped around.
My guess is that the load balancer tried to invalidate the header while preserving the TCP checksum.
