Okay, this is odd thing that happened to me. I'll post up a save pulling off thi bug, er...tomorrow, unless someone would like to try to make it happen again off of this one.

Had a stack of guys on a hill next to an enemy city. Go to attack with the stack (as a whole group, not individual units), ENTIRE GROUP fortifies instead. And they don't un-fortify unless I break apart the group, attack individually, or wait a couple of turns and try again.

Any ideas?

This happens if one or more of the units in the stack are out of movement points.