"Client lease expired" error on server

While running Jonathan's test case today, a server crashed with the following fatal error message:

1450838596.972453669 WorkerTimer.cc:342 in workerThreadMain ERROR[2]: Fatal error in workerThreadMain: client lease has expired, thrown at UnackedRpcResults::checkDuplicate at src/UnackedRpcResults.cc:263

This surprised me, because I thought client leases were only for clients. Why would a server have a lease of its own?

Unfortunately, no other stack trace information was available from the crash.