Designers are not programmers
Apparently, designers shouldn't be programmers - should, in fact, actively suppress any programming ability they have when considering user interface design.
Personally, I disagree. Design is in large part about tradeoffs, clarity/information density/whatever. (I just write the code.) If you're not aware of the technical constraints of what you're trying to do, you'll not be able to make intelligent tradeoffs, you'll just have to guess what's possible.
For instance, my designer is Matt Jones, who is never fazed by trivial implementation details.