Happens all the time. Typical scenario goes like this while on stage:
Bl: "We're doing a new song tonight."
Me: Concerned look on my face.
BL: "Don't worry, it's only 3 chords."
Me: Concerned look on my face.
BL: "The chords are ..........."
Me: Concerned look on my face.
Then while performing the new song:
BL [looks over at me and yells]: "Oh yeah, there's a bridge!"
Me: Concerned look on my face.
Bl: "We're doing a new song tonight."
Me: Concerned look on my face.
BL: "Don't worry, it's only 3 chords."
Me: Concerned look on my face.
BL: "The chords are ..........."
Me: Concerned look on my face.
Then while performing the new song:
BL [looks over at me and yells]: "Oh yeah, there's a bridge!"
Me: Concerned look on my face.