RPGMemes

Honestly even if everyone agreed to a linear story, they can jump the rails without even knowing unless you have clear and explicit communication. DMs should be willing to say "hey, you can do this, but just so you know I never considered this action and might need to make up some nonsense on the fly or take a break to do some new prep" if they think it's necessary. Clear communication beats hiding behind the curtain for the sake of immersion every time.

I was a player in a campaign a while back where this basically happened - we all knew the DM had plans and thought we were following them, but he revealed at the end that pretty much the last half of the campaign had been panicked improvisation of material that he wasn't happy with, because at one point the NPCs we'd been traveling with got on a boat to a new continent and invited us along, and in absence of other clear plot hooks we said yes. Apparently all the prep was on the previous continent and he riffed a ton of interactionless filler descriptions, a random dungeon, and a half-baked new plot, rather than saying "you can go with them but to be clear you'd be leaving my prep". In our particular group's case, we would have happily changed our mind on that basis, but even if we'd gotten on the boat we would have been in a position to understand and enjoy the new adventure better knowing that everyone (including the DM) was venturing into the unknown together.