I just found several 500 errors on a four year old site that was PR0'd (former PR 4/5) and completely dropped from the google index/directory several weeks ago. Googlebot still visits every couple of days, but only the home page and robots.txt page. The 500 errors were mostly due to an ad running on several pages where the link had a "space" after the "href=" ". Thinking back this "space" was probably added in the time range soon before the site was dropped.
Is it likely I've found the reason it was dropped?