ok that sounds bad, you should have the option to go back to design, but depending on what point you find that issue, depends on how much time you have wasted?
It's about defining and solving small problems all the way, and avoiding trying to solve big problems.
If you manage to restrict yourself to only solving small problems (THIS is the true challenge with software engineering, in my humble opinion), then you won't ever have wasted too much time if (when) you need to reset.
