If Someone Is An Illegal Immigrant And Then Decides That They Want To Return Back To Their Native Country, What Complications May Arrise... at The Airport, Customs, Etc...

HI,
They can be arrested, either going out, or coming into, the country.
Best wishes.

I doubt if there will be any problems leaving the country. Customs, passport checks, etc are done when entering a country, not usually when leaving (though some totalitarian regimes do both). If you have documentation of citizenship in the country you are returning to, you should be OK on that end.

I guess the answer depends on how you are looking at an 'Illegal Immigrant'. Illegal immigrant without a visa? Or someone becoming illegal as immigrant after the visa expires?

I don't think there will be any problems at all at least in the second case. But, it will be at least difficult to come back to the foreign country in this case.

In the first case, well my guess is that you won't have problems at all. But can't promise here.

If I were you I would try to find out the policies, regulations anonymously from the concerned institutions.