Due to a race condition in alerts, discrepancies between /alerts and /pools can arise.
$ curl -s http://localhost:9090/alerts search_9200 - Could not depool server elastic1051.eqiad.wmnet because of too many down!
However, everything is fine with the host, both pybal-wise and in ipvsadm:
$ curl -s http://localhost:9090/pools/search_9200/elastic1051.eqiad.wmnet enabled/up/pooled
An old patch from @Joe seems related. I've included it in PyBal 1.14.0, currently being tested on pybal-test200[123]. We'll see if it does fix this issue; the current workaround is restarting pybal.