Hier mal ein Auszug aus einem AVR-Datenblatt, zu finden unter unbenutzten Pins:

The simplest method to ensure a defined level of an unused pin, is to enable the internal
pull-up. In this case, the pull-up will be disabled during reset. If low power consumption
during reset is important, it is recommended to use an external pull-up or pull-down.
Connecting unused pins directly to VCC or GND is not recommended, since this may
cause excessive currents if the pin is accidentally configured as an output.

... bis jetzt wohl immer Glück gehabt