Dota 2 is either the most popular or second most popular PC game on the planet, depending on whether you ask a Riot fan or a Valve fan. Dota 2 Reborn is its attempt to shed the old engine and update everything to be prettier, run better, be more modifiable and allow for better expansion in the future. It initially entered a beta state back in June and has recently received an update to allow Ranked Matchmaking. Unfortunately, there are still various issues, including one discovered today via Reddit that allows the enemy team to have almost perfect knowledge of where you're moving.
The way it works is thus: pre-made teams can have coaches assigned to them that act as observers but can only see your movement. It's designed for particularly hardcore players to have a friend watch over them and try to help them out, remind them about certain timings and generally critique and try to improve their play. Unfortunately, a bug in the Reborn client is allowing them to see usage of teleportation scrolls - a vital tool for repositioning around Dota's map - on both sides. This, as you can imagine, is a little overpowered and means you should probably avoid team queues until Valve can get it sorted.
It's hardly the first of Reborn's bugs, but does seem to be the most repeatable with the largest effect on gameplay. A quick browse showed me there's been some pretty interesting things over Reborn's life cycle, from problems exclusive to Mac:
To fun uses of its custom game testing tools:
And then, well, this:
She has become death. The Assassin of worlds. All will be consumed by her hellish glory.
I've also heard tales of headless characters when skins haven't loaded in properly, invisible attack animations and games lasting so long the map begins to set itself on fire almost at random - though that one is present in the main client too. Valve were probably hoping Reborn would be ready for use in the just-past TI5. At this rate, it might be ready for use in 2017. Bang on Valve time, really.