Create this page:
Blah blah.<ref name="foo"/> ==References== {{reflist|refs=<ref name="foo">blah</ref>}}
Open the classic source editor on the "References" section, and attempt to preview the page.
Result is:
Request from xxx.xxx.xxx.xxx via cp1085 frontend, Varnish XID 45833706 Error: 503, Backend fetch failed at Sun, 12 Jan 2020 22:53:39 GMT
Alternately, try: https://en.wikipedia.org/w/api.php?action=parse§ionpreview&title=foo&text={{reflist|refs%3D%3Cref%20name%3D%22bar%22%3Ebaz%3C%2Fref%3E}}
Result is:
{ "error": { "code": "internal_api_error_LogicException", "info": "[XhujygpAMD8AAJykxxoAAACK] Caught exception of type LogicException", "errorclass": "LogicException" }, "servedby": "mw1228" }
This may be related to T240248.
(Originally reported by Dominic Mayers at enwiki Village pump)