I have on several occasions benefited from either system error or human error saving me hundreds and sometime thousands of dollars.
The thing is, I was too afraid to share these on OzBargain, purely for the fact that this error would then be rectified, stopping me from benefiting from it.
Back in the old dick Smith days, they had a gift card sales, I purchased gifts cards for $1000 to buy a TV but then decided against it and cancelled the order before it was processed. I had to follow up customer service to get a refund which I obtained but then a couple of weeks later I receive the gift cards loaded.
There is another one too recent to share :-D
By 'System Errors', do you include incorrect prices when items scanned at a check-out?
I once had a five-pack of socks scan for $2. Couldn't believe it, and the check-out assistant just shook her head and took my money.