I finally figured it out.
If you are getting this error, the first question is, do you guys run your game through Steam? If so, then this is the issue.
If you remember, setting up the game through Steam requires you to overwrite "Arma2OA.exe" in the Arma 2: Operation Arrowhead root folder with another "Arma2OA.exe" from your Expansion\beta folder. (If you use DayZ Commander, ticking "Replace original Arma2OA files with Beta so Steam works" in the settings will have the steps automatically done for you.) Because you've updated, the Arma2OA.exe in the root folder (the executable that Steam launches the game through) is outdated which is why when you try to join a server, the version number might be outdated even though Steam/Six Launcher/DayZ Commander/everything shows that you're on the latest Arma 2: Operation Arrowhead version. (At the time of this posting, most of our DayZ clients show version 95248 when it should be 95417.)
The fastest fix is to turn off the "Launch using Steam" option in your launcher (Six Launcher, DayZ Commander, or whatever).
If you still want to want to run the game through Steam, then you'll need to overwrite the main client in Arma 2: Operation Arrowhead root folder with the one in the Expansion\beta folder, just like the first time when you setup your launcher to run DayZ through Steam. Here's the default folder locations if you've forgotten:
Arma 2: Operation Arrowhead root folder
Where the old version client that should be overwritten is
C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead
Arma 2: Operation Arrowhead Expansion\beta folder
The latest version client is found here
C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\Expansion\beta
Note that DayZ Commander makes it convenient for you by doing all of the file overwriting for you when you tick "Replace original Arma2OA files with Beta so Steam works" in the settings.
If you're not running the game through Steam and you're still getting this error, well, you probably genuinely have a different version of the game from the server. That either means the server is running an outdated version of the game, or you are. Check your Arma 2, Arma 2: Operation Arrowhead, and DayZ versions.
Edited by BulletEater, 30 July 2012 - 06:15 AM.