Here is a command you never should have on your server:
/agree . This command is used after you joined a server for the first time. You join the server and it says welcome or whatever the admin made it say, and than it says pleas read /rules.
Many guest's don't care about reading the /rules [I alway's do.] and just begin to play without knowing the rules. If that happen's a lot, you can download a plugin to your server that you have to type /agree as a verification of the rules.
When you have read the /rules and typed /gree, it says "Command failed". That mean's that the command don't work. When you cant /agree, you can't use command's and you can't build. The only thing you can do is to speak normal in chat, use colors in chat (you type %[1-9]or[a-f][The message]) and walk around.
So never get /agree, just make the newcommers type every 2nd rule in chat.
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Noticed by Raymondhas, also known as The_Ms and martinh96.