As a developer you should be thinking through your edge cases more than a tester would. The alternative is needing more testing.
Ideal situation: tester doesn't find anything.
Ideal situation: tester doesn't find anything.
Sadly, most of the really good ones are developers-in-potentia, and move out of the QA role after a year or two. Those who really are meant to remain in QA are gold - if you find them, don't ever let them go.
Maybe your software is perfect, but maybe my customer is happy to pay me 4 times less for the software, even though it still contains the few bugs my tester overlooked.
i.e. apple just recently decided to break 5 years of responsive design practices by ignoring the maximum-scale meta tag. their decision is not without merit, but suddenly many web applications started behaving weirdly and encountering usage pattern they were never tested on before.
those edge cases never existed before, but I bet only the shops with dedicated testers caught them within the first week.
"It is impossible to make anything foolproof because fools are so ingenious."