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:
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.