I had to insist on overpaying to get into Corfe Castle in Dorset once. It was excruciating!
XH and i had borrowed my parent's Nat Trust membership cards and were pretending to be them to save a bit of £ getting in to see the ruins 
In my defence DDs were 2, 4 and 6 and money was very tight. We were hoping to just pay for the three kids and us get in free. We were skint and on a very tight budget caravan holiday and this was the only 'pay for' activity that we could afford using the cards.
A very enthusiastic young Nat Trust worker pulled us out of the queue and asked if we wanted to quickly sign DDs up and get them Nat Trust cards for free. Special deal.
Well, a) we didn't know what my Dad's password was or anything and b) couldn't prove 'our identity' because it wasn't us ! and c) if we got the kids cards they'd have to have a different surname which would have been odd ...
All this flashed through my mind so i said
''OH ha ha no thank you, it's fine we'll just pay for the kids!!''
He was 
''But ....''
thinking we hadn't understood
''it'll take just 30 seconds for me to just enter it all on the computer in here and they'll get their cards and get in here free today and all NT properties for the next x years!!''
Me ...
''Oh, ha ha, no really - we'd really rather just pay for the children .... ho ho .....'' 
And we stayed in the queue and payed for the kids.
Oh god. I mean ...
When he walked away he looked soooooo confused. Poor bloke. And i felt like such an idiot!
That's what you get for fiddling the system i guess.