3 years ago I write a point of sale and inventory management system for this shop I work for; created a manual and integrated help and even showed them how to use it.
2 Weeks after deployment:
"YOUR system is screwing up! It's not adjusting inventory after a transaction!!" Screams irate boss. He continues, "Everytime I sell more that 1 thing with your program, it doesn't change the stock; I thought you knew what you were doing??".
I calm him down and ask he to show me exactly what he's doing. He types in a UPC that I havn't seen, turns out it's one of hundreds he's added that reference multiple other items by description alone (eg "3 wigets for $5"). I very politely and calmly explain why the system won't work this way, and show him a very similar, but correct way of doing it. His reply:
"NO!! YOU'RE WRONG! If I know what that means, I KNOW a program can know it too!!"