I waited in the queue (it was benfit day), and eventually it was my turn at the window. "Can I have twenty-five first-class stamps and a 43p stamp please?" I asked. The woman behind the hatch worked out 26x25 using pencil and paper, and got the right answer : £6.50 (I knew this already : one sad thing I've picked up from doing physics is that 5^4 is 625!).
She then turned to a large desktop calculator to check her sum, and typed in '26+25' by mistake. A puzzled look spread across her face. "Surely she'll realise what she's done," I thought. Going back to her pencil-and-paper sum, she tried again, and got another different answer (something like £2.30!). She tried the calculator again, made the same mistake as she had before, looked extremely flustered and fed-up, mentally added the 43p and said : "That'll be fifty-one pounds and forty-three pence, please."
"No it won't!" I replied, and explained the mistake to her. To give her some credit, it had been a long, hot, tiring day!
But still .....