In a machine gun, tracer bullets are phosphorus bullets spaced at consistent intervals in the clip. They allow the shooter to see where their bullets are going. Pragmatic programmers use the “tracer code” analogy as a method to get quick feedback on whether an software approach is going to work (you’ll have to read the book for a more complete explanation)
Testers can also use “tracer bullets” to test better. I think exploratory testing is a great example of this. Some amount of exploratory testing should be done before completing the test design spec (aka test plan), and definitely before writing formal tests. Taking time to see how a component works before writing formal tests virtually guarantees better testing. If (and when!) testing starts before the development on the component is finished (or started), explore the specification. Read the spec and understand the goal. Fire your tracer bullet by paraphrasing the spec for your peers. This exercise gives you the basis for your test design spec, and ultimately, your testing.
Testing doesn’t necessarily start at point A and end at point B. It’s an iterative process, and there are definite gains to be had by getting to point B early and often.
Now tasting: 1999 Folie a Deux Eschen Vineyards Zinfandel.