It's not a true error, because your server does serve a page for that URL. Most likely someone has a link like that on a web page that googlebot indexed, and when Google requested that URL from your site, your server gave them content with a 200 response.
My guess is that someone is using software created for some common webmaster tasks - link building, rank checking on competitors, something like that - and the results page gets published online where googlebot can spider it.
I'd suggest you configure your server to 301 redirect any URLs with query strings you don't use to the same URL without the query string. That will avoid any issues with Google.