Yellow army VS Red army
http://www.youtube.com/watch?v=nHxcnpcXagc
I dont know if this happens to you guys before. But check it out! Enemies are fighting with their friends!! The only difference is that some are in Yellow uniform and some are in Red uniform.
Oh I thought you were going to talk about the mission with Francesco, this is new to me
Remember how in Bonfire of the Vanities Savornarola's men and Borgia's men fought? The Florentine guards (yellow) still attack Borgia troops (red) afterward if they see them attacking you. But both will attack you as well.
This is probably due to what SAndC said but sometimes you get the odd guard fighting another during fights anyway. Maybe they feel sorry for you being so outnumbered. Not that it matters as the fighting is stupidly simple anyway 
I wonder how will the Green army will react..
I wonder how will the Green army will react..
By playing Risk with them of course! 
Thanks to SBIzokronus for the fantastic sig!