Real maximum current for ATmega328?
Summary:


You must distinguish between "guaranteed operating conditions" and "absolute maximum ratings". Also between current from an eg a logic high output pin at a usefully high voltage and short circuit current from a pin.
At 80 mA you are exposing the IC to conditions that exceeds the manufacturer's guarantees for product survival and the manufacturer explicitly advises that such practices may cause permanent damage to the IC.

YMMV :-)




Operating and Absolute-maximum figures

Manufacturers publish data that tells you what conditions they guarantee a device will meet in practice when operating normally. They also publish absolute maximum ratings for a device, beyond which damage to the device may occur. 

On pages 519 and 520 are tables that specify the voltage and current output conditions which Atmel guarantees. Not that as current increases the voltage drops due to increased voltage drop across the internal circuitry. They do not specify what current you can get when you load a high output pin down to almost 0 Volts - but you can be sure it would be more than the maximum guaranteed figure and that it would probably risk damaging the IC. 

The most important specification with respect to your question is on page 317 of the ATmega328 datasheet

This says 

29.1 Absolute Maximum Ratings*

DC Current per I/O Pin ................................................ 40.0mA

and 


NOTICE Stresses beyond those listed under “Absolute
Maximum Ratings” may cause permanent damage
to the device. This is a stress rating only and
functional operation of the device at these or
other conditions beyond those indicated in the
operational sections of this specification is not
implied. Exposure to absolute maximum rating
conditions for extended periods may affect
device reliability.


"Absolute Maximum Ratings" are in all reputable data sheets and mean just what they say. They are the absolute maximum at which the device is guaranteed by the manufacturer not to suffer permanent damage at. Usually the guaranteed operating conditions are lower than the absolute maximum ratings.

You say that "you have tried this on every pin. Note the manufacturer's comment 


Exposure to absolute maximum rating
conditions for extended periods may affect
device reliability.


Here "extended periods" is at the manufacturer's and Murphy's discretion.



Chances are you have not damaged the IC. But if you operate it at above maximum values you may. And if you operate it at above maximum operating values you may get misoperation in practice. "Proper" designs must always observe operating limits set by the manufacturer.