I got a ton of not found errors in Google Webmaster Tools, but the weird thing is, all of the pages ARE found. There is absolutely no error in the URLs they list that supposedly don't exist. Why is this happening?
That could indicate that your site had some sort of technical problem for a while that is now resolved. Examples might include:
- DNS pointing to the wrong server (which returns 404 errors for your URLs)
- A webserver misconfigured for a while to point the site to the wrong directory
- Files on your site deleted and then restored
If those errors are no longer happening on your site you can mark them as "fixed" in Google Webmaster Tools and they shouldn't show up again unless Googlebot encounters them again.
You should probably check a handful in Fetch as Google to make sure Google can find your pages. This will serve as reassurance that pages are accessible before using the MARK AS FIXED button to clear out the errors.
Due to any number of configuration/DNS issues it's possible that you can see pages that Googlebot can't. A service like What's My DNS? can help you determine if you're getting different DNS results than others.