About 10 years ago, one weekend I drove up to an ATM in Upstate NY, and found an elderly couple standing there, looking confused. After a minute, I stopped the car and got out. Both were well-dressed (probably coming home from church); the gentleman was about 90; his wife was a similar age.
"Is everything OK?" I asked.
"Could you help us?", she said (he was too proud to ask for help, I guess).
"Sure! What can I do for you?" I replied.
"You see, the bank sent us this card and said we should be using it, instead of going to the teller inside. But we don't know what to do with it."
Wow. I was shocked. The bank (HSBC) had not even told them how to use the card. And I was surprised that there existed people who didn't know how to use an ATM in this country!
So I showed them where to insert the card; told him to enter his PIN (while I looked away so I wouldn't see it). I advised them about the security implications; and how to always collect the card after you're done (those ATMs kept the card for the duration, unlike newer ones where you just swipe).
I was left with a sad feeling after that, which I can still remember. If I'm ever designing software for general use, I always think about that couple and try to see it from their eyes.