An odd one I've come by is that Elephants can be used as offensive units, at least with Hidden Nationality. I've haven't gone nuts testing it, but I think it's a glitch with open borders.
If you Elephant has Hidden Nationality and you have open borders with the person you are attacking, by moving the Elephant 'over' the unit you initiate combat, although the combat odds are not displayed. Through this I've been able to 'attack' "allied" cities with Elephants. I've also been able to assault their workers as well
I haven't gone crazy testing this, I only just noticed it in a game I was playing just now (had an Elephant walking through my "ally's" areas with HN, racking up some XP on some weak Warrior attacks, when I stumbled upon the fact that I could trample over their workers and attack their cities). For all I know Open Borders has nothing to do with it, but I was always under the impression that basic Elephants weren't meant to be able to attack, only defend.
Edit: Done some quick testing with my autosaves.
Open Borders is irrelevant. You can attack allies with or without it. However, if you declare war on them you can't attack. An amusing exploit, you can attack other civs with HN 'phants at peace but not at war.
If anyone wants to see for themselves, I've attached a save with this. My HN 'phant is parked outside the Lurchiurp capital, prior to cancelling Open Borders or declaring war. With reloads you can see that whilst at peace, attack is possible, but at war you can't.