I've only ever booked two mistake fares.
One a J fare to LAX with Delta IIRC plus a J ARN-EZE one with AZ.
They were obviously mistake fares especially the AZ one which was laughingly cheap. The AZ one had a flight change a few months down the line so when I agreed to the change and it was confirmed I knew it was going to be okay which it was. As for the DL one....a week after booking they told me to go whistle which I was happy to do as I didn't really want to go to LAX.