If a client compiled with ES5.3.2 or later is connected to a server that is a newer version, it might see a runtime error such as:

 

This is a recoverable error, so users can continue past it, and it means that a new message type has been added to the api, and this client just got a message that it did not know how to decode.

 

The easiest fix is to recompile the client using the matching client api.  If you do not want to have to recompile the client each time you upgrade the server, add this line to the client (or similar syntax, depending on client language) and recompile once.  After that, the client should ignore unknown messages.