in reply to
Larry vs. Joel vs. Ovid
i think they're both right.
larry's point is quite valid when you're dealing with just two interacting components. particularly if one of those components is a human (you're asking for trouble if you give a human a text box for input and say something like "just don't put any semicolons in there or it will break things") or otherwise not under your control.
joel simply points out that it doesn't scale very well to systems of more than two components.