[robertgraham]

My guess is that your original SYN did not go to the target, but was redirected somewhere close by. I'd look at the TTL value in the IP header of your first SYN-ACK, and play with such things as traceroute.

Such redirection is often done on a specific port basis, so that trying to access different ports might produce a different result, such as a RST packet coming back from port 1234 with a different TTL than port 443.

There is so much cheating going with Internet routing that the TTL is usually the first thing I check, to make sure things are what they claim.