Test, Test and Test Again
snikkersnak writes "Richard Collins has written a piece about developers and testers; the article is arguing that in closed development these two roles have to be chained together one-on-one in order to reproduce the 'release early and often' effect of open source development."
That's the kind of testing you can actually do fast and close to the development team, but the real problems come from the impact your code could have on parts you barely know they exist in the full system.
Parts testing is just step 0 of the testing process.