A situation where propaganda could be dangerous would be a political leader using propaganda in order to incite violence or dangerous ideas into people's minds. One example of this would be Hitler, as he used propaganda in order to incite violence and the killing of the jews.
Propaganda can be used for good, however, if it is used in order to inspire people to help out their country. An example of this would be WWII, as people used propaganda and posters in order to inspire people to help out the war effort and to help out the soldiers in the fight against Germany. This was beneficial as it gathered more support for soldiers fighting in the war and helped out in the long run.