Way back in October 2002, I had the enviable position of ramping up the development effort for the Sun RI for JavaServer Faces. At that time, Test Driven Development (TDD) was just starting to catch on, and I used my position as team leader to mandate (HA!) that we would use TDD on the project. I realized that for any mandate to succeed, it must be easy to implement, so the team and I invested in building the infrastructure iteratively growing our test codebase. I can't tell you how many times that investment has paid off, but I'll tell you one time: right now.
While implementing Adam Winer's proposal for fixing the State Saving Window Id problem, I came across a corner case not covered explicitly in the proposal. Basically, the proposal re-allocated some responsibilities and neglected to re-allocate this one corner case, which happened to be caught in an HTMLUnit test, written by Craig, I think. Unfortunately, the net effect of this neglect was to throw the webapp into an infinite loop. It was very easy to re-produce: just visit the prefix mapped webapp and include only the servlet-mapping with no trailing slash and no viewId. Thanks to the unit test, I quickly found and fixed the problem, without introducing the bug into the code.