What is error guessing, and how do you use it?

Error guessing is an experience-based technique: I pick tests based on the mistakes developers typically make, not only on what the spec says.

What I typically try

• Text: empty, spaces only, very long, apostrophes like O’Neil, emoji, <b> tags.
• Numbers: 0, negative, too many decimals, other number formats (1.250,50 in Germany).
• Dates and time: 29 February, another time zone, midnight, daylight saving changes.
• Actions: the same thing twice, interrupting an action halfway (lock the phone, take a call).
• Environment: permissions denied, slow or lost network.

How I make it reliable

1. I cover the spec first with equivalence partitioning, boundaries or decision tables, so coverage is measurable.
2. Then I timebox an error-guessing session, often as part of exploratory testing.
3. I keep a checklist fed by real escaped bugs, and pick items by risk for each feature. A planned version of this, where you list likely failures and design a test to provoke each one, is called a fault attack.

The weakness is that it depends on experience and its coverage is hard to measure. That’s why I combine it with systematic techniques instead of relying on it alone.

Learn more: Error Guessing and Checklists