Why does a mains appliance give a shock after it is completely unplugged
First, this is not a fault, but perfectly normal and safe (if slightly unpleasant!).

The large brushed motor in a food processor can generate small sparks and electrical noise (interference to radio signals) when it is running, so a small capacitor is connected across the AC mains to suppress this noise. 

If you unplug the appliance when the AC voltage is at its peak, the capacitor is charged, and can hold its charge until you touch the pins.

It is a low value capacitor (typically 0.1 uF) so the charge stored should not be dangerous. (It is also a special construction - usually "class X2" so that if the capacitor breaks down it will fail safe instead of creating a short circuit)