Module 6: Error Guessing & Anti-Patterns Checklist

A senior tester walks up to a form they've never seen and immediately knows where to look. Not because they read the specification. Because they've seen the same mistakes hundreds of times before.

Error guessing is not luck. It is a learnable skill built on systematic knowledge of how developers go wrong in if/else blocks, conditional chains, and input validation. This module teaches you to build that knowledge deliberately — and to encode it in a checklist that makes you effective even on your first day with a new system.

Lessons in this module

1. Error Guessing as a Professional Skill — What error guessing is (and is not). The difference between ad-hoc guessing and structured, experience-based prediction. How to use bug taxonomies and defect databases to focus your attention on the highest-yield test areas.
2. The Developer's Top Logic Mistakes — A guided tour of the most common conditional errors: off-by-one in ranges, = vs == confusion, zero-as-falsy gotchas, missing else branches, incorrect Math.abs() assumptions, and the forgotten trim(). Each is a test case waiting to be written.
3. Building Your Anti-Patterns Checklist — How to capture, organize, and apply a personal checklist. What makes a good checklist item (actionable, specific, falsifiable). How to keep it alive as you gain experience.

Practice: Trick Form

A form with no documentation and three of the most popular developer mistakes baked in. Your job: find all three using nothing but your professional intuition and a checklist.

By the end of this module, you will earn the Bug Whisperer badge.