Designers unintentionally make things unnecessarily hard for users. Designers naturally know a lot about their systems, so they under-estimate or ignore what may become design problems for users. Designers can think and reflect about their systems in a relatively relaxed development environment where many things seem obvious, but it is a completely different environment from that of the users, such as nurses, firefighters, pilots, drivers, and others with demanding work that leaves reduced attention for dealing with design oversights. This article gives some motivating examples, and makes suggestions so we can recognize the key problem, the curse of knowledge, and consider ways to mitigate it.