I've had several overbooking experiences with CX.

The most interesting was Sept. 14, 2001 HKG-YVR. (I'm pretty sure it was the 14th - could have been 15th) I was lucky enough to book a mileage ticket on this flight on 9/11 and I turned out to be right - it was the first flight from HKG to north america after 9/11. As you could imagine, many many people wanted on that flight and I was just lucky to have a res. I offered my seat to CX but they were not interested. I flew it.

Another was SFO-HKG a few years back. I think they offered something like US$1000 for an op-down from J to Y plus a ticket refund. I'm pretty hazy as it's been a while but I'm sure there was money - don't bank on my details. They didn't end up needing my ticket.

I'd say maybe 5 - 10% of the time my BP spits back out and CX fishes for my new upgraded BP. (I'm on a slow streak) I suspect most of these are OP related. I'm AA EXP and cross the pacific every month on CX in paid J.