I think he got offended because of Mike's tone of voice after he made a sarcastic joke (the I have no idea what your talking about).
Ians known for his dry humor and sarcasm though, I'd think Mike would have seen through that and just let it go.
Either way I still found it funny
. Hope we dont get some random dude when we go to Brooklyn. I dont mind Ian and Jim Spanarkal being our main guys at first but I hope we can get someone well known.