Ah yes, the dread all too sudden move in the seat. In such a case the only reasonable thing is to get up and stand for the rest of the ride. Worst case you might just have to walk home. The damage is already done.
I remember crying all the way to the next stop as a kid because I was too awkward to let my seat neighbour know that I needed to get up to press the stop button. In defence of the bus and its inhabitants everyone was very supportive. Good times!