It’s quite common and i’ve never known its meaning.
If you’re considering it’s appearance these days as “common”, fuck knows what you’d have called it august 08 to early 09. You couldn’t escape it then.
I remember it back then. You couldn’t join a server without getting it a few times.
Anyway, a snapshot is where the server takes the current infomation about everything on the server, and sends it to you. If theres too much, it sends a reliable snapshot overflow.
He means you shouldn’t consider it common today, as a few months to a year ago it happened like 3 times whenever you tried to join a server - if you were lucky.