I've tried this on many other systems and ports without the same behavior. It's really looking like just a bug to me. Any more ideas on what could have set it off?

On -T5, it took 1.4 seconds (does the time column in ethereal represent seconds?) for the first syn/ack to respond to the first syn, 1.2 for the second syn/ack.

On -T1 it took .7 seconds to respond to it. I think the double SYN's from -T5 may have made it delay, possibly syncookies or some other TCP stack protection delayed it enough where nmap couldn't read it?

Does the -T5 option send out multiple syn's per port for you too? Is that normal and expected?