1. Have a simple MessageBox:

   QMessageBox msgBox;
   msgBox.setText("Hey yo!");
   msgBox.exec();
   

2. Build and execute this:
   
   No surprises here.
3. So far the MessageBox has only one button. Let's add another one:

   QMessageBox msgBox;
   msgBox.setText("Hey yo!");
   msgBox.addButton("Reject", QMessageBox::RejectRole);
   msgBox.exec();
   

4. Build and execute this:
   
   Huh? Still only one button?
5. Let's show the MessageBox before adding the button...

   QMessageBox msgBox;
   msgBox.setText("Hey yo!");
   msgBox.show();
   msgBox.addButton("Reject", QMessageBox::RejectRole);
   msgBox.exec();
   

6. Build and execute this:
   
   So now there are both buttons??

When I see a function named addButton, I expect it to add a button and nothing else. Above all, I don't expect that this removes a button.

If there's a good reason for this behavior, then it should be documented clearly. Right now, this is not documented at all.

I don't think that the behavior of addButton should depend on whether show was called or not.