Friend of mine just divulged his 'money saving' strategy to fly 'flatbed' whilst paying lowest based Y fares. He does this on KLM.

He basically books his (cheap Y) ticket and two fully flexible Y fares for family members. He assigns the seats so they are in a row of 3. KLM has no penalty for last minute cancelations on these tickets, so he cancels the checkin and the ticket whilst boarding in process.

So far he has done it 5 times and KLM seems none the wiser.

It is of course very bad of him, but it seems to work.

(In fairness, I am inclined to buy a fully flex and cancel for my next flight on KLM to HKG to make sure the seat next to me is free...)