Finding out there's going to be another baby in this house next year.
While being incredibly furious about it. 
I know the feeling.
Several years ago, my wife and I were discussing how we wanted to be addressed when we became grandparents. We talked about the standard Grandma and Grandpa, the French Grandmaman and Grandpapa and even discussed the Danish terms (they differ depending upon whether the person is the father's or mother's parents).
Anyways, the next day (almost like she read our minds), my oldest calls up and tells me that she is pregnant (thus beginning several months of being refered to as my idiot 17 year old). When my wife got home, I told her that my oldest had called and said "Hi Grandma".
My wife didn't speak to me for 6 hours. And then, because
I was there, I got yelled at for 1/2 an hour.
That was about 13 years ago. We just had both of our daughters' sons with us for a couple of weeks. Love them both to the ends of the earth.