Uneven heat distribution in oven. See that vent is unobstructed
COOKIES TOO DARK ON BOTTOM
Cookie sheet set on too low a rack in oven
Cookie sheet too wide or too long for oven
Uneven distribution of heat in oven
MUFFINS HAVE TUNNELS AND SHARP PEAKS
Overmixing
PIE SHELL SHRINKS
Pastry stretched in pan
Too much water