Don't feel bad. It is gonna happen...
I have done it a bunch of times. I try to keep cool, but sometimes I slip up and it is expected.
Once we had an exchange about him taking one of our kids because the other one was sick and the well one was bored and called him. He was going to come get her, but then said something about "oh, to be clear you want me to take her on YOUR day?" I got freaked out thinking he was going to try and use it against me somehow so I said "forget it." Then he texted me 3 x saying, "let me help, please, what did I say, etc?" Then he sent email telling me I am putting the kids in the middle and using them as messengers, blah, blah, blah. I did not respond to any of them.
Later that night after a hard, hard day with the kids I was having a glass of wine. I texted him back saying, "Don't worry about it, I have taken care of sick kids plenty of times while you were out fucking whores." I shouldn't have done that...but oh well. Shit happens when someone shits on you.