How are regression bugs more dangerous than other kinds of bug?

Here's how I'd rate the "dangerousness" of a bug:

  1. How easy is it to detect?
  2. How easy is it to reproduce?
  3. What are the consequences of it occuring?
  4. How likely is it to happen?

Look, bravo for all the TDD. TDD eliminates a huge chunk of bugs. But by definition, the bugs that you find with CI are easy to detect, easy to reproduce and 100% likely to happen. Sure, without a TDD/CI system, these bugs may not have been detected, may not have been easy to repro. But the reverse does not hold: a TDD/CI system doesn't make all bugs easy to detect and easy to repro.

So all the other bugs that your system has right now, are the ones that are left: hard to detect, hard to reproduce, and don't always happen. Now turn on a thousand users. How many users are you hoping to have btw?

Your worst kind of bug:

  * Is not detected for months.
  * Unable to reproduce.
  * Company killer. (Reputation, lawsuits, whatever).
  * Happens once every 40,000,000 sessions.

Not detectable using TDD and CI. Company still dead.