If your site is having such problems like Google crawling the non-existence page, then make sure to control over this issue otherwise your site can affected in Google Search Result Page. Because Google considered this as duplicate content. You might use 301 redirects for all and canonical references even a number of URL’s are being crawled in GWT (Google Webmaster Tools).
As in my point of view your need to return a 404 to your server for any URLs which doesn’t exist. Certainly your site loses Ranking in SERPs periodically or loses of traffic rather than of returning a 404 it’s appropriate in such case. You will be getting WMT warning regards “soft 404”. But this way you can control over this issue and can prevent your site from being penalized or drop in Ranking in SERPs.
