Snowballing. It can determine a game completely, and even lead to a huge comeback to a team that had most of their lanes destroyed. In order to better understand the concept of snowballing and the champions that do so the hardest, I'm going to talk about what I feel are the worst offenders and if any of the other users know anything I missed, or feel I'm wrong about, please comment.
Jayce: When you play vs. Jayce, and your top starts dying to him, you basically can just sit back and wait for him to come for you next. I'd assume the shock blast/acceleration gate combo is what really gets a lot of people who aren't used to having to dodge skillshots for their life as a top laner, or maybe the complaints of him being broken are far beyond the modest outcry for his nerf. What I do know is that whenever I play against a Jayce, I find myself fighting a brutally destructive fighter that can shoot you to death, but can still survive and stay in the fight if you attempt to focus him. I really, really hope I don't have to hear the JayACE joke again after some player completely dominates his lane and brings his sweeping power down to the other lanes after, but I know I will, because either I get bad top players often, or something is incredibly broken in this champion.
How to counter: I have no idea how to counter this champ. By the time he's gotten to me, he usually is far to strong to touch. Luckily, high ip cost means you don't bump into this guy too much. For the next champion, I can't say there are any of the same restrictions.
Master Yi: Before the rework, he was very annoying and took little to no skill. now, he's very horrifying and takes little to no skill. Master Yi puts the power of a king in the hands of a unskilled player oftentimes. Also unlike Jayce, almost everyone has bought him, or will probably buy him someday on a whim. This means any player, even those who rarely play melee champs, has the potential to wreak a team. Plus, he can be played in almost every role, similar to Jayce. The reason for his snowballing to me is due to his buff, and the fact that ult passively lowers cooldown for his other abiilties if he gets a kill. The fact that he already can speed up the cooldown of Alpha Strike by doing the exact thing he will be doing the whole time he is snowballing: hitting things. This essentially means that every kill he gets makes him stronger, but not in the leveling to 18 while everyone is 14 sense, but the fact that he can immediately pull the same combo on another champion. This doesn't even include the fact that his ult has an increased duration for each kill, which is even more fuel to the flame and means that unless you also have a speed buff (almost any speed buff with a long duration will outrun Highlander, or you're a champion that has a cc besides a slow, you can expect yi to stick to you incredibly hard and destroy most of your team even if you run immediately after the teamfight starts, which causes him to snowball even more. Besides that, he can also meditate under tower or when he's in a tough situation if you try to kill him before it's too late, and in the case of the tower, only careful timing will prevent him from simply healing under tower and then killing you while you're weakened from the tower shots. He really does destroy basically everything if he's rolling.
How to counter: To start off, never say cc counters a champion, especially in yi's case. A better statement is that good timing, range, and skillful aim of ccs besides slows counter yi. When yi is in highlander mode and fed, you'll want to escape as soon as possible. Unless he's about to Alpha Strike you, yi will always be running straight for your throats when he's using highlander, or running directly away from you if he's running. You can use this to your advantage by using a straight shooting skillshot such as Ahri's Charm, or a targeted snare, stun or any other ability that stops yi in his tracks and wastes his ult. The reason why I don't mention aoe skillshots is because with that increased speed also comes the ability to run right through aoe when you aim it, or even run over it before it takes effect, like Nevermove or Gravity Field. This is why I say cc besides slows can't exactly stop Yi, because not only are the number of them that even do something besides slow are medium at most, but besides that, you also can subtract taunts and debuffs (though debuffs can help you peel him down fast enough to still win the teamfight if your team decides to stand their ground) So you end up with snares, stuns, and Charm, which as I remember still applies the slow to yi with highlander because it doesn't count as a slow.(yet another benefit of one of the heaviest ccs in the game) Whimsy may also bypass the slow prevention, but I haven't played vs a yi as a lulu, nor own her, so you'll have to take that from your own experiences.
However, if you feel you can hit a hard to hit aoe, even in consideration of the superspeed, go ahead. Unless you're Anivia. If you are and feel that yi is going to sweep over you, pinging for a retreat and then immediately getting to the nearest chokepoint and walling is is the best option, because if you wall your escape route your whole team is dead, and if you try to escape too late, he'll likely run past it as you cast. In fact, if you're Anvia against a fed yi you should be in a more defensive position as his ult nullifies all your damage. But besides that, if you have a huge gamechanger ultimate that can waste his duration use it. The use of your taunt, pushback, or dance-inducer can save your whole team, even if it wastes just a little of the duration.
If you want to shut down yi early, your best plan of action is to try to bully him a lot before level 6, as he won't have his super escape, and he won't have the big items yet. When doing this, you should still bring your jungler along, because even an unintentional Alpha Strike can allow him to live just long enough to kill you, and double strike can give deceptive damage. If you're ranged, harassing him a lot is important, because yi can't do a lot to you without getting close up. So, if you were playing for example Urgot, you'd want to abuse the fact that early on you can harass him with your eqqq without much effort as much as possible. However, you should also make sure to wait for his meditate to be down, as well as Alpha Strike, before you do anything. If not, he can either dodge your skillshot and waste your mana or heal himself back up with little ability for you to defeat him while he is unless you can out damage his healing or nullify it. So as a top laner, Executioner's Calling would be a viable build item if you think you really, really need to beat him out of lane. If you don't do that, you can use the same tactic used against an under tower anivia egg or Aatrox revive: Kil them, then kill them again while it's on cooldown. However, you have a far smaller window. Alpha Strike will come back up pretty quickly unless you deny him any basic attacks, and meditate gives you maybe 30 seconds or so that you can tower dive him and not get denied. If you get them both down and can go in on it, you'll likely get the kill. However, don't try to keep going for a kill if someone was just killed by yi past level 6, because that means he can quickly make a turnaround on you. So another good tip is to strive as hard as possible to prevent any deaths by yi, which includes escaping when low during a gank by your jungler, helping whoever needs it the most at the time instead of the carry, and essentially not playing recklessly or trying to do more damage and dying instead of getting out. Because for yi, the more enemies, and the faster he can strike them each down, the stronger he is.
I really have more I could say about countering yi, but it's getting late, and I'm trying to save up for Shaco. So comment on any snowballers I didn't mention and any flaws in my strategies, and also mention some of the lightest snowballers (I'd for one say Ahri, as you can dominate lane and be insanely fed but still not be unstoppable) if you could, as I haven't seen enough games to conclusively mention certain champs (which is why Poppy, Vayne, Kassadin, etc.) Aren't on here.
Btw, Fun Fact: Syndra can use Force of Will to pick up Zac's bloblets. Also, throwing the Ancient Golem restores mana, and the Lizard Elder applies the buff's on-hit effect. This ignores the fact it probably can also pick up Zyra's plants and Yorick's ghouls, but some interesting tidbits besides the obvious Tibbers nullification technique
Have a nice day/night!