I've gotten $3k ecredits accidentally a couple times. However never an unlimited "Amex" basically.

In my case issue arise when buying a ticket from AF/KL and using PayPal for payment. Then DL takes over ticket at some point and reissues on 006 stock. If I then cancel and request refund (refundable fare or still in 24hr window) AF/KL process a refund to PayPal. However the DL system doesn't understand a PayPal refund done by another carrier. Thus DL system eventually shows an ecredit for the price of ticket.

I called in and talked with ticketing support even. They appreciated my honesty, but there was nothing they could do. The DL system couldn't see a refund, so accounting wise it had to remain as an unused eticket available to me. The agent even went ahead and booked new ticket for me. In all I've had 3 tickets this happened with, coming out to tune of $9k approximately.