On our install https://phabricator.wikimedia.org/ if you search for "help" it 503's. I played around with debug limit trying to get a stack trace and have not been successful yet. In testing on the upstream I notice that searching the 'help' literal seems to have specific behavior. My naive and probably ignorant view of this is that is may be related to:
We do not have diviner installed locally, and I think that is more common than not for self managed installs.
503 details from our reverse proxy:
62 FetchError c Resource temporarily unavailable 62 FetchError c backend write error: 11 (Resource temporarily unavailable) 62 VCL_call c error deliver 62 VCL_call c deliver deliver 62 TxProtocol c HTTP/1.1 62 TxStatus c 503