What process do you use to test and find bugs in an application you've developed?

"Since I typically use the agile or waterfall development processes, just as you do here at Stripe, I test my application at various stages during the development process. This allows me to correct errors before they have greater implications later in the process. I use a variety of tools to test and debug the software. I also ask my peers to review my work during the development stage. Once the application is completed, I send it to the quality assurance team for final testing and debugging."