lucy24 - 9:07 am on Sep 23, 2013 (gmt 0)
Are you asking about the 404 page itself? Sure, slap a "noindex" meta on it if you're worried. But I don't think search engines index error documents; they probably don't even read them.
:: detour here as I realize I've got a very easy way to check ::
I have a stylesheet that's used only by my error documents; almost nobody sees it except humans who got locked out by mistake. afaik the googlebot has never asked for it-- that's what I just detoured to look up-- though they routinely get my other stylesheets. That can only be because they don't even look at the 404/410 page, but just note the header. (The plainclothes bingbot constantly asks for the file, but that's just part of the act.)
Don't block error documents in robots.txt; that could be counterproductive. Besides, robots don't normally ask for your 404 page unless there's been a mistake of some kind. They get handed it whether they want it or not.