Wireshark version 1.12 is out, so I changed the meaning of “tcp spurious retransmission” info message to make it easier to find what you’re looking for. “Spurious Retransmissions” means that data was sent again that the receiver had already acknowledged. It means that the sender thought packets were lost and sent them again, even though the receiver sent an acknowledge packet for it. The change Sake introduced to packet-tcp.c was to mark repeated SYN and FIN packets as retransmissions, which wasnt the case before.”]
Source: https://blog.packet-foo.com/2013/06/spurious-retransmissions/