The Broken Window Theory

The 'Broken Window' is a theory in criminology regarding urban disorder and vandalism

Quoted from wikipedia :

Consider a building with a few broken windows. If the windows are not repaired, the tendency is for vandals to break a few more windows. Eventually, they may even break into the building, and if it's unoccupied, perhaps become squatters or light fires inside.

This translates very well to  software development, as soon as minor smells are introduced, these 'Broken Windows' start to multiply until there is massive backlog of neglect that need to be fixed. 

Quoted from Coding Horror:

Don't leave "broken windows" (bad designs, wrong decisions, or poor code) unrepaired. Fix each one as soon as it is discovered. If there is insufficient time to fix it properly, then board it up. Perhaps you can comment out the offending code, or display a "Not Implemented" message, or substitute dummy data instead. Take some action to prevent further damage and to show that you're on top of the situation.We've seen clean, functional systems deteriorate pretty quickly once windows start breaking. There are other factors that can contribute to software rot, and we'll touch on some of them elsewhere, but neglect accelerates the rot faster than any other factor.

 Places I stole the quotes from :
http://www.codinghorror.com/blog/2005/06/the-broken-window-theory.html
http://en.wikipedia.org/wiki/Broken_windows_theory