We knew it already but now Google confirms. Pages which give errors such as 404 or 500 eventually drop out of index and lose PageRank and are unable to accumulate or pass any search engine signals.
“If a URL becomes an error, for example a 404 or a 500, we will try to keep it indexed for a very limited period of time. After that, we will drop them as they aren’t useful for the users. For the URL this means that with its content it will lose everything it had before. PageRank can only exist for live URLs that have a representation in Google’s index and thus URLs that were dropped because they became an error, can’t pass signals.”, Gary Illyes (Google).