All general I/O-pins have internal ESD protection diodes to GND and VCC, as shown in the figure below. If exceeding pin voltage 'Absolute Maximum Ratings' in the datasheet, resulting currents can harm the device if they are not limited accordingly. For parts with LCD-driver, the same situation on SEG pins used for general I/O can also influence the LCD voltage level.
Figure 1. AVR I/O Pin Protection
