Some smartphone-optimized websites are misconfigured in that they don't show searchers the information they were seeking. For example, smartphone users are shown an error page or get redirected to an irrelevant page, but desktop users are shown the content they wanted. Some of these problems, detected by Googlebot as crawl errors, significantly hurt your website's user experience and are the basis of some of our recently-announced ranking changes for smartphone search results.
Starting today, you can use the expanded Crawl Errors feature in Webmaster Tools to help identify pages on your sites that show these types of problems. We're introducing a new Smartphone errors tab where we share pages we've identified with errors only found with Googlebot for smartphones.
Oh, nice. So now my 403s on midi files are listed twice: not only for all robots generically*, but then all over again for robots representing a UA that can't play the midi format anyway. (This is not hypothetical. I checked.)
* I'll lift the block when and if they add a "Sounds" tab to the search options. Not before. What do they think, that I've got juicy .html files hidden behind the .midi extension?