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:
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/search/engine/PhabricatorJumpNavHandler.php;c6a7bf87294d28d52587e62b2ad740cd4c6ca32e$7-10
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```