/effect @p 5 30 255
hit some mob
profit

Launcher output:
Client> 2013-05-30 10:43:37 [SERVER] [INFO] <Zambonifofex> LOLOLLOL
Client> 2013-05-30 10:43:37 [CLIENT] [INFO] [CHAT] <Zambonifofex> LOLOLLOL
Client> 2013-05-30 10:43:54 [SERVER] [INFO] [Zambonifofex: Given Força (ID 5) * 255 to Zambonifofex for 30 seconds]
Client> 2013-05-30 10:43:54 [CLIENT] [INFO] [CHAT] Given Força (ID 5) * 255 to Zambonifofex for 30 seconds
Client> 2013-05-30 10:44:01 [SERVER] [INFO] <Zambonifofex> xdxdxXXDX
Client> 2013-05-30 10:44:01 [CLIENT] [INFO] [CHAT] <Zambonifofex> xdxdxXXDX
Client> 2013-05-30 10:44:35 [SERVER] [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
Client> 2013-05-30 10:45:01 [SERVER] [INFO] <Zambonifofex> trollolololololollooollool
Client> 2013-05-30 10:45:01 [CLIENT] [INFO] [CHAT] <Zambonifofex> trollolololololollooollool

i said something on chat as "landmarks"

LOLOLLOL=before using /effect

xdxdxXXDX=after using /effect

trollolololololollooollool=after hiting a cow

it's weird, it's like if the local server had stoped, but if you wait some seconds, the game turn back to normal

Sorry, i'm brazilian, and my english is not good