I've come to realize that, if you construct your application in proper iterations, you can do QA testing at any phase of a project. Sometimes I just don't have the spare time to dig into the codebase, implement features, and keep up with goals and progression, but I'd love to contribute, even if it's at a distance. Sometimes you will find overlooked bugs that are so glaring that you don't need knowledge of the deepest, darkest recesses of the codebase.
But if you keep finding hard bugs, people won't like you. ;)