It is easy to get a very messy structure that makes getting an overview of the codebase really hard. Great care must be taken with names, repetition and general structure to ensure the maintainability of a project.
Just because it works on your machine doesn't mean it works on everybodys machine. Different browsers have differing capabilities. Ensuring you don't use functionality not available on all browsers you target is both importment tricky. This is doubly true for the browsers on mobile devices.
Chrome dev tools are very useful for debugging.