Something that sounds like it will be incredibly awesome but lets you down. Comes from an experience with Wells Banana BreadBeer. Sounds great, doesn't it?
Man, that Nuclear Death Star Firework sounds great! Let's light it up! ........... Damn, it's a friggin' banana breadbeer - 10 seconds of fountain and no boom.
To avoid/ignore/not notice the main part of a conversation, topic ect.
Originating from presumably by someone putting away shopping, and a loaf of bread falls out the bag and to the floor. But the person does not notice it/ignores it and continues to put away the shopping.