Ran into an error a few times before the 0.6 update, and then ran into it quite a bit right after the 0.6 update. Haven't had it happen much recently, and I don't remember the exact wording of the error. I'll add a screenshot if I see it occur again.

Not sure if you'll be able to get the exact error, but here's the savefile, mods list, and logs: https://drive.google.com/folderview?id=0ByjNoH5l8fatVjRVQU1UTXktN1k&usp=sharing

Thanks for the report, I think I tracked it down to a rare case where a biter hive that was just generated by the map generator was marked as 'under attack', and then had the error with the plan. I am hoping 0.6.1 fixes it, let me know if you encounter it again.

Not fixed. A good way to reproduce it seems to be starting with a 'Very Small' map.

Yep. I found it in one of my own saves, in fact! I just uploaded version 0.6.2. Hopefully that is the end of this particular bug.