Happened to me, once. Had accepted a VDB for a redeye LAX-JFK flight (oversold in J, got offered a very nice voucher, hotel, dinner, and first flight out the next AM in F). Was just waiting at the gate for them to finish boarding, so I could get my hotel and dinner vouchers printed. For some reason, they ended up with one free seat in F, and there was some issue with non-revs (I think the GA messed up, and didn't realize there were non-revs, and shouldn't have issued the VDB compensation). So, the GA says "go ahead and board, take the one seat in First, the FA knows what's going on." No boarding pass, no nothing. Got on, FA points me to an F seat, sat down, and we left. The VDB voucher was still valid, so I essentially got the VDB voucher and an upgrade to First. Never did get the miles for the flight, but decided that it turned out well enough I wasn't going to bother and risk getting the VDB ($800 if I remember correctly) voucher cancelled, or get the GA into trouble.