Played with a really negative fizz, he bragged about he wouldn't be banned, reported him, received this message a little later.
http://i.imgur.com/Q5x3cM1.jpg
EDIT: GGWP, Got another one! Really impressed with riots new system.
Justice 
I saw one of these and didn't read it correctly and thought I was the one being punished :(
So you're supposed to see this if one of your reports is successful? Lol I've never seen this in my life so... none of the players I've EVER reported have ever been punished?
Yep, I've seen that thing a lot. I tend to hold my reports and only report seriously toxic peeps, About 1 in 3 times I report I get the message after the fact, lol. Victory to super high reporting accuracy making reports more powerful!
all on foreign servers.... i've never had that message appear in my 5 years of playing.................. Like come on. 
Had about 4 of those already :P oh and 
Justice takes wings!
I've only seen it once myself, but goodness was it satisfying to see!
I played a game with a really toxic varus last week. he was probably my least favorite kind, the really passive aggressive one that acts like hes done nothing wrong. After claiming I would probably get banned soon (never been punished) and that he did nothing wrong (which was false), I reported him and as soon as I left the game, I got one of these lovely boxes. It was the first one I'd gotten since I don't usually end up reporting people.
I've seen this many times. I'm guessing it happens when someone gets close to 9 reports in the single game.
Pure justice 
*
Got another one...
This is wonderful. An increase of these messages can really help reduce the thought that "reporting does nothing", which in turn will increase the chance that neutral players will send out reports
Plus it makes you feel good when the naughty are punished >:D
Yea from what I have heard OCE is a cesspool of toxic players
Best of luck in your future games
Justice has arrived in LoL
This seems to be happening more lately. Probably due to riot upgrading the system that detects toxic playstyle and chat recently. Glad to see it working. I don't shed a single tear to the players who received well deserved bans after they show the reform card.
