I get a 'client.dll mismatch' error when joining a game server.
If you get an error about a "client.dll mismatch" when you connect to a server, this means that your version of the "client.dll" file for that particular game doesn't match the version that the server is running.
This error is more typical for 3rd party mods rather then for the regular Steam Games.
If you're playing a third-party game (i.e., one listed under "Third Party Games" in your games list), make sure you have the latest version of the game. The Server you are are trying to connect to might be out of date too.
If you get this error on a regular Steam Game, it's most likely that the Server is out of date, try a different server.