Here's a true story from Father Time (Lee Sklar) as published in Bass Player's Session Legends & Studio Gear, pp. 34-35):
I used to have a bass with a 'producer switch' on it. That's what I called it. All I did was drill a hole and install a toggle switch; it wasn't wired to anything. When the producer would ask for something different, I'd flip the switch and he'd go [gives the thumbs-up sign]. "That's great!"