This happens to me about 80% of the time. The error appears right at the end of the map loading screen—when it says, "Loading client..." I get a popup: "ERROR: Reliable snapshot overflow".
So I have to reload the map (which takes much less time than originally).
This happens when I choose a server in the server list (a map in progress), and when I am already on a server and there is map change.
What could cause that? How can I solve this?