The only thing we do with RSS feed is to create new ones manually on /admin/plugins/rss_polling. Nothing else.
I have noticed that the order of RSS feeds in that page changes from time to time, which is already strange and it doesn’t help debugging problems because the chronological order of feed creation is lost, and the new sorting doesn’t seem to follow any alphabetical or numerical sorting.
We don’t know how often this problem happens because we cannot detect it right away when it happens. In a best case scenario we can only wait for the owner of the feed to tell us that new entries are not appearing, or we realize ourselves that, say, a weekly podcast has been silent on the forum lately.
I have the impression that this problem might be related to situations where the CPU gets close to 100%, or something else in the server reaching maximum capacity. Even if the scheduler does a good job at polling only 5 feeds at a time, on the hosting analytics we can see that when RSS feeds are being updated the CPU basically reaches 100%.
Maybe this is why forums with just a couple of feeds don’t notice it, but if you add plenty something might break only sometimes regardless of the scheduler?
See related: Are there any upper limits to the RSS Polling plugin? - #4 by icaria36