Cool: submitting my first patch to Lucene, to repair a minor bug
Embarassing: Finding out that I had a bug in the submitted code. It would be the one time I didn't run the test suite because I was in a hurry, of course.
A learning experience: Getting a very nice response from a developer which actually taught me a lot about Ant in the process. Of course, it helps that he wrote the book on that subject :)
