My personal favorites:
(At the least, this one halts escalation)
I hadn't thought of it that way before.
(This one basically tells the other that my position is not the only position)
That came out all wrong.
(Easy one to correct a misstep. Also good when you want to say something anyway but get away with it
I see how I contributed to the problem.
I can see my part in this.
(These two are always the silver bullets for me when needed.