Does your site have any 404 errors? Are you sure?
A 404 error - or a broken link - essentially means that particular page is no longer available, which causes an error, which causes user frustration, which impacts your bounce rate, etc.
So how do you do that? This infographic from Spiralytics provides an overview of the impacts of 404 errors, and tools that can help you find and fix them.
As noted, unless you're absolutely sure, it could be worth a check-up.