Subject kinda says it all.
I have seen and experienced issues where your passport is no good for travel within six months of its expiration date.
Why?
I mean, if that has to be the case for some reason then why not just move the expiration forward six months and make that the end date?
Travelling on an out-of-date passport is usually illegal, and always a bureaucratic nightmare…
So the 6 month time limit is to make sure that your passport will still be valid for your return trip.
Most countries won’t let you in if your passport is about to expire. They want you to leave, ya know.
If an airline carries a passenger to a destination where he is not allowed to enter due to invalid paperwork, the airline has to pay the expense of carrying that person back to where he came from.
Think about how pissed off the airline would be if they had to give a free ride to a non-paying passenger and his two suitcases. 
Why isn’t that the problem of the person traveling?
It’s up to you to be certain all of your travel documents are in order and that you do not overstay your welcome.
From what I’ve heard: During those six months, the passport is still valid, so you can still use it as identification, such as when applying for a driver’s license. But for other purposes, “they” are worried about the fact that it will expire soon. For example, when traveling to another country, the foreign government wants to be sure that you’ll have a valid passport when you head home. So even if you’re planning to return in two months, if your passport is expiring in four months “they” will worry about the possibility of you extending your stay. “They” decided that six months is a useful and simple cutoff date.
(“They” can be lots of different things, in different situations.)
It is the problem of the person travelling, but in any case the border policeman doesn’t have to let you in.
“why isn’t that the problem of the person travelling?”.
See the last two sentences of my post above…
Yes, it’s your responsibility.
But if you screw up,you don’t pay…it is the airline which has to pay for taking you back home.
And they can’t collect their beloved fees for your luggage…
But if your docs are not in order, or if you do overstay (even through no fault of your own, like an extended hospitalization) “they” will have to deal with the bureaucratic nightmare.
That’s how they make it your problem: by simply barring you from flying or entering the country with a passport that’s close to expiration.
I see no reason the airline can’t charge a person for a flight home…even a forced flight home.
Ok.
But why not make your passport expiration six months earlier than is printed on the passport?
In that case there seems to be a problem involving infinite recursion. (your passport expires six months before it expires)
Yeah…kinda silly.
It’s not that they’re no good, it’s that they’re less likely to be good. Plenty of countries will let people in with less than six months remaining; whether the airline will accept that is another matter.
Actually…thinking on it more…doesn’t an expired passport mean you cannot get back into your home country since you no longer have valid ID that you are a citizen of theirs?
If so then how can an airline be forced to fly you there for free?
BTW in my experience, fixing irregular travel and residence paperwork problems is not a fun way to spend your time, honestly I cannot recommend it.
The computer knows all. Losing a passport or having one expire does not wipe out your records. But there were rumours that somewhere sometime some fake refugees conveniently lost all their papers so they could not be reliably identified or distinguished from real refugees and sent back.
They can, and they will. The point is that the question whether you fly back or not does not hinge on whether you pay or not. Many countries have a law that says that foreigners who are not allowed entry into the country have to be brought back to their point of departure by the airline that brought them there. Non-payment of the fare for that return flight is no defence on the part of the airline. So if you’re refused entry, the airline cannot tell the authorities of that country: “Sorry, we’re not hauling that passenger back; he doesn’t have a ticket, and it’s not our problem if he’s stuck here The Terminal-style.” Instead, the airline will haul you back. But they will, subsequently, try to make you pay for that flight. Whether that succeeds or not depends on your financial situation.
Makes sense but what if I fly to Germany on a passport that has five months left on it and I stay six months?
Who has to fly me back for free and deal with collections?
What if I travel there on a passport good for eight months and stay nine? Does that change which airline has to fly me home for free?
No. The one thing that a country will do for its own citizens is to make sure they’re not stuck helplessly abroad. If you arrive at the airport of your own country with an expired passport of that country, you will be interviewed by immigration officers to find out if you’re really a citizen and thus entitled to enter. There are many ways of finding this out other than presenting a non-expired passport. The process may be a pain in the rear, and it may lead to you being fined for violation of a law that requires you to have a valid passport upon entry; but they will not simply turn you away and leave you to fend for yourself.
Wasn’t there a guy stuck in a French airport for years because he was a person without a country?
Why do you assume showing up at the airport with an expired passport is a reliable way to score a free airline ticket? You would probably get in trouble over the irregularity and have to buy a ticket anyway.