We’ve all worked with testers who were bad at their job; it was frustrating and felt like a giant waste of time and effort. But hold on to those bad testers: they’re more useful than you think.
In my experience, bad testers have several common traits: they’re fairly illogical, disorganized, forgetful (and never write things down), very bad with computers and think they can work without understanding what they’re working on. In other words, bad testers are like bad users.
Which is why they’re so handy: they will run into bugs no competent tester will ever be creative enough to test for, but that bad users will always find. They forget so easily (and have nothing written down) that they always run into first-time-user issues (normally UX bugs). They don’t quite understand how things interact, so they flush out missing back-end protection against wrong combinations. They find ways to exploit every security vulnerability you have because they don’t like remembering log-in details, favour re-using things rather than creating new ones (such as shopping carts, orders and registration forms) and always push session tracking to its limits. They show where your UI is messy by getting lost in it. In short, they stumble around knocking things over and showing which ones can’t bounce.
So if you have a large enough testing team, keep a bad tester around. It may not feel like it, but they have a lot to contribute.