Currently I have a wordpress site submitted in Google Webmaster Tools. But at the Crawler Errors page, I see a lot of : site.nl/post/feed. Does anyone know how these get there and how to get rid of em?
Those are 'pretty permalink' style Feed URLs. Google doesn't guess at URLs, so your site must be printing them somewhere. Or, your site used to print them and you've since disabled your feeds or broken them somehow. Google would then have those URLs in its database.
You have to make sure your site isn't mistakenly printing bad URLs, then, if your site is no longer printing the bad URLs, they will drop out of Google's database eventually. You can set them to 'fixed' in webmaster tools but they will show up if Google finds the link again.