In Logstash (Perf Team dashboard) most the messages are from failed attempts to acquire a Memcached lock (presumably due to TKOs and link saturation etc.)
It's not very frequent, but whenever it happens, it logs the following twice for the same reqId/url:
2020-05-16T19:31:49 ERROR memcached mw1395 Memcached error for key "rl-deps:skins.minerva.base.styles|minerva|ne:lock" on server "127.0.0.1:11213": A TIMEOUT OCCURRED 2020-05-16T19:31:50 ERROR memcached mw1395 Memcached error for key "rl-deps:skins.minerva.base.styles|minerva|ne:lock" on server "127.0.0.1:11213": A TIMEOUT OCCURRED
This suggests it's attempting the lock twice, which seems contrary to what the code says. A bug perhaps?