As of 22:00 UTC yesterday (2013-11-15) our servers suddenly started getting "Unknown authorization header" failures when connecting to Google Calendar API (V2) via OAuth.
The problem seems completely intermittent. Right now we retry after each failure twice (with a four second delay each time) and approximately 10% of these retries succeed.
I saw on this page that Google engineers monitor this tag on stackoverflow. If that is you - please help! My service is suffering and our customers are losing business. Please help me convince them they were right to bet on us and Google Calendar!
I managed to get in touch with a Google SRE. They have since diagnosed the issue and they are pushing a fix now. Should be fixed sometime in the next few hours, I'm told.