I had an issue this week on a system which has been working fine for a while, but stopped fetching some data from google's user account API. I was getting a 403 response from the API, which seemed odd. Luckily I was logging OAuth::getLastResponse() to my error logs (this is PHP code, and you need to call OAuth::enableDebug() before you make the request to get this output) so I could see that I was getting the following back from Google:

GData
sslRequired
SSL is required to perform this operation.

Closer inspection shows that for one of the google endpoints, I had a prefix of http:// rather than https://. Those single-character bug fixes that take hours to find are my favourite!