Regression Testing
Regression testing is the repetition of previously run tests after changes have been made to the source code. The purpose is to verify that things in the new build still work according to specification and that no new bugs have been introduced in the intervening coding sessions. Although it is impossible to quantify precisely (some have tried), figures that I have come across from time to time suggest that for every ten bugs that are identified and cleared, perhaps another four will be introduced. This sounds like a realistic figure to me, although I would apply it more to process code rather than event handlers, which are more self-contained (which, of course, is a benefit of the object-based model that Microsoft Visual Basic employs). As you continue each test/debug iteration, the overall number of bugs in the code should decrease correspondingly until a shippable product exists.