Has anyone else experienced any problems when stacking two MakerShields together? I'm working on prototyping my first Arduino project (a Sonic Screwdriver sort of goofy thing), and when I tried stacking two MakerShields, I suddenly started having a problem with Pin D9 (or maybe it would happen on other pins, I don't know):
MakerShield A plugged in alone: Piezo buzzer on D9 makes the sound it should make when button on D1 is pressed, LEDs on D3-D6 light up one at a time, which one selected by button on D2.
MakerShield B plugged in alone: Identical results.
MakerShield A plugged into B, all stacked onto the Arduino: LEDs work properly, but total silence from the Piezo.
What's going on here?