Working on a crossword puzzle with ink means two things: you find yourself more conservative, (You wait until you are fairly confident before committing to something.) and when you inevitably make a mistake anyway, it gets messy to patch things up.
Software development is the same way. No matter how carefully a developer proceeds, at some point a mistake will happen, and if the erroneous code has gone out the door, you have a messy situation that needs patching. At that point, software can begin to look like a badly scribbled-out crossword. Depending on the location of the error—whether it's in a fundamental API or just an implementation detail for instance—external developers can be affected as well, making their code messy too.
-- Micah Dubinko, Life After Ajax?
