I've heard numerous stories of people accidently messaging their opponents with text they intended to put inthe Notebook box.
It can be quite embarasing and ruin a good game.
I've also made this mistake 3 or 4 times.
It happens because the 'Message Your Opponent' and 'Notebook' buttons are right next to each other and work in almost identical ways.
If the programmer/designer of the interface screen moved the 'Notebook' button to a place lets say underneath the 'Skip this game for now' button it would never happen.
I know the people that run this site have lots to do, but it would be a good idea to move the two options away from each other in a future release of the game screen interface.