Over the past week or so I have noticed very occasionally that a page or an asset has not rendered in response to a request. choroba has also mentioned in the CB seeing this or something similar. Just now this has happened fortuitously while I had the network console running in my browser and it reports these problems:

A 500 response when attempting to retrieve https://www.perlmonks.org/css/common.css from the server at at 11:16:54 GMT (Response header timestamp).

A 500 response when attempting to retrieve https://www.perlmonks.org/?node=list replies toggle javascript from the same server at the same time.

The 2 assets were then immediately re-requested by the browser and successfully served by that same server with the same timestamps.

I hope that this helps in tracking down the problem.