The stoner
Background : This player can be tough to deal with if you aren't familiar with stoners. A reasonable chunk of the MOBA community likes to smoke a bowl before they play. Fewer play ranked games stoned, but it definitely happens, and it can be a problem. Its good to identify this guy as early as possible, so he can be "handled" as needed. An opening chat like "WAZAAAAAAP" or "WOOOOOOOORD" is usually a pretty good indication. You can also outright ask if he's stoned, if you are suspicious. He is unlikely to lie about it. A lot of these guys will straight up talk about being high in chat. Don't assume its some kid trying to look cool, its probably just a stoner talking about being a stoner. There are a few important things to keep in mind about stoners. First and foremost, they are likely to have pretty bad tunnel vision. You can expect them to lane well, but map awareness is basically out of the question. They are likely to work their way into dangerous spots without noticing, and botch ganks in their lane due to not noticing your jungler is there. Keep in mind that they can likely focus just as well as a sober player, but their multi tasking ability is greatly reduced.
Solution : Ping. A lot. Seriously. Ping everything. If your jungler is in his lane, ping 3 times. If their jungler is in his lane, ping 3 times. If you expect him to do anything at all, other than sit tight doing exactly what he's been doing for the last 30 seconds, ping it. The audio cue from the ping noise is the most likely way to get his attention. You should also keep in mind that he may not be reading chat at all, due to the focus issue. Just ping, trust me, its the best thing you can do. Try to offer helpful advice when possible, and warnings if he is being careless. He is likely just failing to notice, rather than being consciously reckless. If he is reading chat, he will probably get scared and tighten up his play; for a few moments at least. Try to maintain good vibes if he is talking in chat. His performance will likely be tied to his mood, and if the experience turns sour, he may just leave.
Stock Response : "Watch it dude, you're gonna get jumped up there" / "Are you high, son?"
