Heat oven to 300 degree F. Remove all unpopped kernels from popped popcorn. Combine popcorn, cereal, pretzels, and pecans in a 17x12x2-inch baking or roasting pan.
Mix brown sugar, butter, and corn syrup in a medium saucepan. Cook and stir with a wooden spoon over medium heat until mixture boils. Reduce heat to medium-low. Cook without stirring for 5 minutes more.
Remove pan from heat. Stir in pumpkin pie spice, baking soda, vanilla, and red pepper. Pour over popcorn mixture in pan, gently stirring to coat. Bake for 15 minutes. Stir mixture; bake 5 minutes more. Spread caramel corn mixture on a large piece of buttered foil to cool. Break into pieces. Store tightly covered up to 1 week. Makes about 8 cups.