You forget to seal and vent the piecrust
THE RESULT: Overflow.
THE FIX: Venting and sealing are crucial pie-making steps. As the pie bakes, steam is created from the moisture in the filling, causing it and the crust on top to expand. Properly sealing the edges keeps the filling from leaking out and creating a sticky mess beneath the pie and in the oven, while the vents in the top allow built-up steam to escape. For added insurance, place the pie plate on a foil-lined baking sheet in case some filling does bubble over.