The Appeal to Ignorance Fallacy is an interesting one: it states that something must be true because it hasn’t been proven false.
This fallacy is often used by people who believe in entities like Bigfoot, the Yeti, or the Loch Ness Monster: they will say that no one has proven that Bigfoot doesn’t exist, therefore he must exist! With an example like this, it’s very easy to see the false logic.
The same kind of fallacy is common in software testing as well. Consider this statement: “We know our software is secure because we’ve never had a security breach.” Having no security breaches does NOT mean there are no vulnerabilities in the software. It is possible that there are dozens of security holes in the software, but the company hasn’t grown enough for a malicious actor to decide they are worth exploiting. Some companies might also say “We’ve never found a security vulnerability in our software.” That might be true, but it could be that the reason it is true is because they’ve never looked for vulnerabilities. It’s bad logic, and bad practice, to say something doesn’t exist because you’ve never looked for it.
Another example of the fallacy happens when someone announces that their company’s app is “bug-free”. This is an impossibility. Lack of found bugs doesn’t mean that an application is bug-free. It means that the testers haven’t found any bugs recently, nothing more. A simple application with just two buttons has at least two different testing paths. Add a third button and you have at least six different testing paths. So imagine how many testing paths an application with a dozen different features could have! There is no possible way to test them all, so there is no possible way to prove that the app is bug-free.
Watch for this fallacy when your team is discussing software. When someone makes a bold claim, ask yourself if it has actually been proven, or if it is merely wishful thinking.