Invalid URLs and Google Rankings
It would certainly seem logical for invalid URLs (broken links, etc.) to negatively impact a site’s broader Google rankings, but according to a recent Google Webmaster Help thread (http://goo.gl/BbtMB), that’s not exactly the case.
The specific question in the very active thread was if “a website had several thousand internal URLs that are broken, can it be a signal to Google to crawl the website less and reduce its trust in the domain?”
Google’s John Mueller responded much later in the thread that the number of crawl errors on a site “generally” does not affect a site’s crawling, indexing or ranking.
“The number of broken links -- assuming the links point to URLs that don't exist -- generally does not affect your site's crawling, indexing or ranking at all, regardless if it's a handful or millions of them. This does not make us assume that a website is of lower quality (personally, it's more like a sign that the website is technically handling these invalid URLs correctly, which would be a good sign). The number of 404/410 crawl errors would also not negatively affect the crawl rate of the website -- it might even increase the crawl rate since the server can likely respond to these requests a bit faster than to normal requests.”
So, in short, while invalid URLs may not hurt a site’s ranking in general, it does indeed negatively impact the specific invalid URL (because it can’t be accessed and thus won’t rank) because the full internal linking opportunities aren’t being leveraged.