When developers test their own code, they tend to check that the code works instead of being creative at finding ways to make the code break. I've always found that having an independent tester makes a huge difference to the reliability of your code. Independent testers try to find ways to break your code and will frequently do things in a different way than you do them yourself. They may use menu items when you always use hot keys. They may use copy and paste where you always use drag and drop. They may do things in a different order than you do. As a developer, it's hard to break out of the way you designed the code to work and think about the way people may actually use your code. A fresh set of eyes always helps break these barriers and improve the reliability of the system.
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