(Also known as Boston Cream Cake and Banana Cream Pie) Boston Cream Pie is a phrase meant for declaring that you have been interrupted while telling a story. The relation between the food and the situation (although weak) is that someone was being interrupted by someone who wanted to talk about pie all the time.
"So anyways, I was went up to her and I-"
"Dude, hold on I need to take this call."
"And then I went back to my room so I could-"
"Would you like a refill of your Sprite sir?"
"Hey, wanna hear something funny?"
"I'm hungry, I'm gonna go grab some food."
"......I guess not..."
These are the situations in which the person interrupted should say:
"Boston Cream Pie"