While taking computer science classes in college, I was taught to distrust a user’s input in all cases. The theory goes that a user’s data can’t be trusted because it could be malicious or just a simple mistake that causes your program to have an error with input it didn’t expect. So you protect your system from incorrect user input and sanitize it. It always felt like one of our dirtier secrets to me, however I fully advocate the practice in code.
My problem comes when the people building software turn from distrusting their user’s input to having a level of disdain for the users themselves. It starts innocently enough with the engineering principles I described above, but can sometimes turn into small things like, “Our users won’t understand that,” and starts to slip into things like, “Our users are dumb, so we won’t do that.” If you’ve started to hate your users, you’ve gone too far. If something is too complex for your users to understand, it’s your job as the developer/engineer/analyst/etc. to make it so that they can understand it. If you think of your users like idiots, your system will reflect that and they will notice.
If your job is to design software for people to use (which, is pretty much all software), make sure you work with the user, instead of against their best interests.