Google Webmaster Tools and Crawl Errors

Hi, I have noticed that while looking at my google webmaster account (not that I really know how to use it) it keeps flagging up 5 Crawl Errors on a weekly basis.

One for example:
index.php?main_page=product_info&products_id=450

The product does not exist, so what could be causing Google to do this?
Each week I mark all five as fixed, but they keep reappearing.

A minor problem I know, but still want to know why.

TIA for any help/comments.

Five is small. The products existed at one time, so Google indexed them. Now Google is going back to find them again and they aren't there - so it marks it as an error.
If you tell Google it's fixed, google thinks that you have brought that page back from the dead and now it can visit it again. So it tries again and there is still no page. That's an error (not a new error - it still counts as the old error but with a new "found on" date.)
I don't know what the "correct" way to let Google know to stop trying to index those page is. I've been told it's already done automatically in the "page not found" part of ZC, but Google still tries to find some of my links 6 months after I sold the product so I just ignore it. If it finds my pages that do exist I'm happy and it can root around for non-existent pages all it likes.

Google removes entries after getting a 404-not-found or a 410-gone response. (They don't remove it immediately, but it does get queued.)
Zen Cart sends the 404-not-found when someone visits the product-does-not-exist page.
Of course, it can only do that if no addons have altered its normal operation.

And of course, if you have links to those products sitting on other websites/blogs/newsletters/whatever and Google is indexing THOSE sites, they'll keep finding your link and add it back to their list, thus canceling the queuing for removal.

ah - I understand now, thank you both.