It's not for a flight, but I once had this situation after a purchase :

I worked hard to save enough and buy a $1000 reflex camera, and I ended up saving $300 more so I added a compact camera to use for parties or bad conditions.

When I they processed my CC at the shop, it showed : "trouble with payment, take your CC back" on my screen, and the cashier told me "no, that's alright peyment confirmed.

After 3 months, (and now 4 years) I was never charged. $1300 worth cameras for free. I eventually bought a $700 lens for the reflex in the same shop, which was charged.

I think I was just lucky that the bug happened on me, and as for the shop, they didn't lose their customer anyway. I don't feel like I stole from them, as it just didn't work and that's life, and it is also a little amount disappeared on the million $ statements of the company, though I understand that it may bring some ethical doubts.

I would be happy to have my free flight one day, but I've had my chance already so I hope it'll happen to others, and possibly when they book premium!