We had expected to have a large turnout for our server open house. 70+ people had clicked on attendance on our facebook page. So we asked the server hosting company if we could upgrade to the next slot up in the packages without it breaking our current setup. (our Mod setup is not a normal modded server)
The answer was sure no problem! so we went ahead and upgraded our server so we could accommodate more users on at a time. The upgrade went okay, the control panel showed more users available to log in at a time. The game client still showed the old setting. We needed to reboot the server for it to take effect. When the time rolled around, I turned off whitelisting to allow anybody on the server. When I tried to log into the game it said that I needed the latest version of Minecraft. I immediately thought no, no, no. They said there would be no issue. Remember I am a programmer not a Minecraft Gamer so modding the server was not exactly my thing. YET.
It was time. A ton of new kids were going to join and my mods were going to show them how easy and simple writing a few lines of code could accomplish the task at hand.
Well the kids got on and we couldn't run anything written on the server. So they played the two hours and I compared the backup I took to the current settings on the server. Turns out all I needed to do was remove a .jar file that was added to the server during the time of the upgrade.
So we extended the Open House for 5 hours after that and they had a blast.