When the backend dies and Varnish generates a nice error page, the footer will contain something like
If you report this error to the Wikimedia System Administrators, please include the details below. Request from 220.127.116.11 via cp1066 cp1066, Varnish XID 12345678 Error: 500, Internal Server Error at Sat, 03 Mar 2018 19:19:49 GMT
which is not all that useful for tracking down the error (it's possible with the timestamp and IP, but cumbersome). It'd be nice to include the XID in the varnish logs (it's added to responses as X-Varnish, I presume to backend requests too).
Especially important for thumbnailing errors as those are the most frequent sources for this kind of error page.