When we testers find yet another "Did you even run this?" bug, it's easy to believe developers purposely inject bugs just to taunt us. I have worked with a lot of developers over the years, and I've found that they generally do try to test their code but simply don't know how to go about doing so effectively.
http://blogs.msdn.com/micahel/archive/2006/01/25/TestingForDevelopers.aspx