Error with my Event?

If you're using OnClientEvent be sure that you're using a local script. OnClientEvent fires when FireClient or FireAllClients is called from the server.

An example of what this interaction would look like:

--Server
local RemoteEvent = game.ReplicatedStorage.RemoteEvent
RemoteEvent:FireAllClients("Howdy")

--Client (local script)
local RemoteEvent = game.ReplicatedStorage.RemoteEvent
RemoteEvent.OnClientEvent:connect(function(arg)
	print(arg)
end)

This guide may also be useful if you need to know more: http://wiki.roblox.com/index.php?title=RemoteFunction_and_RemoteEvent_Tutorial