Buying a Return, Only Using Outbound Leg
 

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This shouldn't be a problem. I haven't heard of anyone ever being invoiced for the difference.

The usual rules for risk minimization apply however:

• Buy the ticket directly with the carrier, not an Online Travel Agent or even worse an independent Travel Agency. At least in the US the airlines charged the difference not to the end customer but to the TA.
• For further risk minimization, do not put in your FB number. I would not bother about it, but it's your call.
• Do not buy the ticket through a company contract with the airline.

I would be very surprised if this is still possible.

As far as I know, this is not possible now.
They are getting quite strict. If you are travelling from Antwerp to anywhere via Amsterdam and the first leg is by train, you still to pick your train ticket in Antwerp otherwise they will not let you on the plane in Amsterdam.

So, it would be the same for Lyon-Paris by TGV with AF. Several times I hesitated buying my ticket from Lyon, since they are 50-100€ cheaper. Apparently, I did the right choice not to buy them.

Isn't this two entirely different things:
a) not taking the inbound flight
b) not taking the first leg of the ticket?

Not taking the first leg of the ticket does definitely not work. However,
I discarded the return flights 3 times in the past 5 years (once even on advice of the KL ticket desk in AMS), always with my FB number in the booking, and I never had any problem.

And yet, I would do it only in emergency situations, you never know if you might get into trouble (i.e. are charged for a one-way flight in the end).

I find that the airlines really cannot impose charges against you (or your TA) if you don't make the return flight. Especially on non-changeable tickets. Because, well, STUFF happens and there are many reasons why you legitimately cannot make that flight and that you cannot change it.

I've had many situations when my plans change and I cannot change my ticket so have to forfeit the return leg. If I can change it even for a fee, I would rather. But then in most cases when change fees allow changes, they usually cost more than the ticket itself. So why bother?

Although it's against the rules, airlines rarely enforce the rule and I know of quite some examples where my colleagues have also not used the return flights in order to cut costs for their departmental travel budget. :D

It does lead to no-shows for the airline if you do not cancel and that in turn makes airlines overbook their flights again.
But I think these full fare return fare policies are really old fashioned pricing mechanisms.
One should be able to book a oneway at a reasonable price and until this is in place, people will always try ways to circumvent the airlines policies.

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Interesting. How much time was between the missed outbound and the taken inbound leg?

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