If you set a port to 0 "from inside" (by program), a MOS transistor switches the pin to ground. Now if you connect to that pin some positive voltage without anysignificant series resistance, current will flow through that transistor, limited by its channel resistance R_DS,on. Whether the transistor will survive this treatment, see datasheet; but if it will,there will be the applied voltage accross it so the input circuit will sense the voltage as logic1.
But be warned, the output transistor is usually not rated for such treatment and you can easily damage it this way.

Jan Waclawek