HA! I figured it out. I'm using an external display over thunderbolt, and that IP looks like it's the broadcast address for the default network that my mac attempts to setup over thunderbolt when there's a connection. I've included a screenshot of the problematic interface. Simply disabling the ...

reproduced:
0.000 2017-01-01 19:40:41; Factorio 0.14.21 (build 25327, mac, steam)
0.000 Operating system: Mac OS X 10.11.6
0.000 Program arguments: "/Users/dwatson/Library/Application Support/Steam/steamapps/common/Factorio/factorio.app"
0.000 Read data path: /Users/dwatson/Library/Application ...

This is the whole log file, I can reproduce again if that will help...

When I load this save for multiplayer, it crashes within about 10 seconds every time.

0.000 2016-12-31 11:10:02; Factorio 0.14.21 (build 25327, mac, steam)
0.000 Operating system: Mac OS X 10.11.6
0.000 Program arguments: "/Users/dwatson/Library/Application Support/Steam/steamapps/common ...