Mess up hreflang?  Google Webmaster Tools might be adding a feature that would show when they feel there might be an error in a site’s usage of hreflang on their pages.

At SMX Sydney Wednesday, Gary Illyes did a session on hreflang and dropped a few tidbits including the possibility of showing potential errors.  The example he used was es-la doesn’t mean “Spanish in Los Angeles”.

Hreflang is one of the areas I know newer webmasters struggle with, as the consequences of making a mistake can be pretty significant and is one of the reasons some are reluctant to start using it.  Google offers lots of information on how to use it here.

I definitely hope this is one that makes it into GWT.

Added: Google does currently show syntax errors from implementation in Webmaster Tools (Search Traffic -> International Targeting).  This would be specifically for errors where the implementation is correct, but they think there might be an error in what you chose for hreflang.)