You mentioned that you had 10K pulldown resistor on the inputs. Clearly then the switches for the inputs must be pulling the inputs high. I suspect your problem is that you are applying a voltage to the input that is more than a few tenths of a volt above the Vcc of the processor. Anytime an input goes above Vcc by a significant amount the input protection diode will turn on and current from the input will flow into the chip in large quantities. It can even cause the chip to enter an SCR latch up mode that can casuse the chip to draw current from the supply till it burns itself up. Under all considitions you need to ensure that inputs do not go above Vcc.

Note there is an exception to this statement for chips specifically designed for 3.3 volt operation but are stated in the data sheet to have 5V tolerant pins. In these cases the inputs can go to 5 Volts since these chips have a different kind of input protection circuit.

Michael Karas