They're always the highest when I have to speak to someone in authority -- passport control, police officers, city/state officials behind glass in offices who never seem to smile. Usually these involve standing in a line for a while. During that time I think about everything I'm going to say and everything I'm going to stutter on, and I have to keep myself from death-spiraling into a bumbling mess. Also I never really know what they might ask, so it's hard to practice anything beforehand.