londrum - 11:09 am on Dec 30, 2011 (gmt 0)
i've noticed a couple of times in the past that Google WMT will suddenly report hundreds of 404s for pages which no longer exist on my server. they were spiderable in the past, but dropped out of the index when i deleted them (months and months ago)
and these weren't the kind of pages to attract backlinks either. so i presume that google must occasionally try and spider non-existant URLs from one of their old indexes.