I had this happen yesterday with opera 27 too. Deleting the google cookies and then restarting the browser fixed it.
Best posts made by threatdown
RE: Cannot load Gmail ... "Cookie Mismatch"Opera for Windows
RE: Tab switch behaviorSuggestions and feature requests
What behavior were you trying to get it to do? If you were trying to make it so that it switches tabs in the tab strip order (instead of recently used order), there is a setting for that:
Opera Menu | Settings | Browser | Uncheck the box for 'Cycle tabs in most recently used Order'.