The forest river outside the Elvenking's halls, reached by going down through the trap door in the cellar, is only survivable in a barrel. If Bilbo enters it any other way - for example by jumping through the trap door on his own - the room is described and then 'you are swept forcefully against the portcullis.', and he dies.

If he is inside the barrel (object 19 is his holder), nothing happens here: he floats on, and two turns after the trap door was opened the barrel timer (A4F4) carries him out onto the bank of the long lake. This is the book's escape in the barrels, turned into a rule you cannot get round.