A Google Webmaster Help thread has confirmed reports of a bug with Google Webmaster Tools. The bug is someone technical, where you try to download the verification file via a URL that Google provides. That URL, which seems to be google.com/webmasters/verification/verification-file-dl?hl=en, is returning a 500 internal error.

Dennis G. from the Google webmaster team confirmed the bug, saying:

I personally see the 500 response code, but honestly, I am not exactly sure how he triggered this URL and if I have to trigger it from a specific page to work or not.

Forum discussion at Google Webmaster Help.