"Have you ever been in a position to definitively tell a user they're wrong? "
Sure , all the time , from lies about have you turned it on and off again? to yes it will work that way , no its not broken , its you etc etc . Thing is , they are the customer and the customer is always right , especially if they are a paying customer. So a little sublty is required , in fact a lot is required to avoid the customer feeling stupid or guilty afterwards.
So much as we'd like to think we are "Put your PC back in the box , take it the shop and tell them you're too F****** stupid to own a pc." types , you probably wouldn't last long in the job if you really did that.