402 Unknown Client

Sometimes you'll see the below error in the Debug log, along with numerous instances of connection failure:

locket: ++ Connect failed, message.successful: false; message.failure: undefined; message.error: 402::Unknown client

I found an article on this subject at this URL: https://success.salesforce.com/issues_view?id=a1p30000000T0F0AAK

Apparently the issue is related to proxy servers. Set the browser to not use proxy servers and test.