Yeah I have the standalone tool and it's a must-have for electric work. While replacing some switches in my house I thought I had the right breaker (sidenote: those breaker-detector tools are like magic, much better than the "is the light still on?" method). The lights went out but when I jammed my tester in the gang box it started beeping. I ended up needing to flip off 2 breakers (took me a while to find the second one) before it finally stopped beeping. From that moment on I promised myself that no matter how "sure" I was, I would ALWAYS stick the tester in before reaching in, even if I hadn't touched the breaker box between working on a switch.
I've been bitten enough times in software alone by things I was "sure" about, I don't take any chances when it comes to electricity.