Scott Mattocks has posted the next "letter" in his LUCID development series (more here) - the I is for Isolation:

Applications are a collection of individual components; they pull together the contributions of individuals to create something which is greater than the sum of its parts. If one piece breaks down, and the system is not prepared to put it in quarantine, the entire application can come crashing down. [...] The best way to stave off this infection is to prevent it at the source. Keep the application happy by making it more resistant to implosion.

He talks about the "code jerks" of your application that hog resources or functionality and cause general problems for the application as a whole. He notes that sometimes it's the result of a third party, but it can also be internal (and more difficult to find). He gives a more specific example of a database wrapper class and some suggestions of things to do as failover (ex. used cached data or read from a secondary).