Saving One: £1.70 or £10 depending on how you look at it.
I very politely explained to the barrier man at Oxford Circus that I'd forgotten to touch in with my Oystercard when I got on the DLR at the start of my journey and I'd have problems getting through the gates. Could he help?
"Where did you get on? So, you're saying the reader didn't work?"
"No, I just forgot".
"We don't get many people like you through here. I should fine you £10. It's nice to meet honest people. Everyone else blames the equipment. Go on, go through. Have a nice evening".
Saving Two: £3.00.
Having worked in shops when I was much younger and having made mistakes when bored, I always appreciated it when a customer pointed out I'd cocked up in case I got in trouble for it. So, when I signed to pay for a book (in a well known bookstore) and saw £4.99 instead of £7.99 I told the guy on the till (much to the astonishment of the people already queuing behind me). The young lad started flapping, clearly not knowing what to do so I very sheepishly said, "I wouldn't want you to get into trouble".
He smiled, "No danger of that. Go on, take it at that price".