An unsecured or incorrectly secured fault was received from other party

I spent a few hours trying to figure out why I get "An unsecured or
incorrectly secured fault was received from other party" after deploying the
app into the staging server.

I turned out that it had nothing to do with security - I added a new method
to contract and forgot to update the service. So client was updated and it
was calling the new method that simply did not exist in the service.

I have not idea why this exception was thrown. Couldn't it be something more
descriptive, "method not found" or something?

InnerException was null, by the way.

And also what was odd was that service died after the first error (it went
to the fault state and was coming back only after iisreset). I have no idea
why this was happening either..