Browsing r/all, taking in whatever was popular at any given moment. This only works on big Lemmy instances with wildly diverse federation.
I don't think the instances needs to be that big to have enough networking effect to get most of all comms. Feddit.dk for instance is relatively small but the all feed is basically identical to any other major instance.
Certainly a very small instance could have a temporary issue like this but it could be easily improved by just fetching the missing comms. And again, you can certainly find smaller instances than lemmy.world that still have all the stuff in the all feed.