Jungler gets spotted
It's really easy for a jungler to mess up a gank. Run through wards, push past minions, or simply come in at the wrong direction and you give your position to the enemy far in advance. Sometimes enemies are blind and you can get away with doing this, but usually it ends up wasting your time. Sometimes it even gets you killed.
There are so many ways to be sneaky when you approach the enemy that there's no excuse for sloppiness as a jungler. Flash is mandatory as a summoner spell for a reason, and numerous junglers have gap closers that let them hop over walls. Lee Sin has more than one!
Sometimes you honestly don't know whether a location is warded or not. At my skill level, it's always a question. I rarely know whether the enemy laners are warding, and there is absolutely no way of knowing whether the enemy jungler is. My only clue is when I see a ward in someone's inventory disappear, and that is ambiguous if the enemy support buys a Sightstone. Sure, you would think that a Sightstone means that there will be wards, but you would be surprised how many people buy a Sightstone and don't use it.
It is not as simple as always assuming there are wards, either. Wards mean that you have to gank from a different (and frequently worse) angle, or that you'd be better off clearing wards instead of ganking. If there are no wards, then you want to approach from the best possible angle. Sometimes laners will give you clues; if you run through a place that might be warded and the enemy immediately reacts, you were spotted and should back off. If you ping to approach, your support can say that the enemy support is warding and you should change your approach or do something else.
Lane jumps the gun
Premature engagement is one of the reasons I ping to signal my approach relatively late. I generally announce my intent to gank only about 5 or 6 seconds ahead of time, even if I'm on the other side of the map and it will take me longer to arrive. I've mentioned a lot that behavior in lane tells you a lot about whether a gank is coming, so any change in aggression by an ally in lane can tip the enemy off that you're ready to jump in.
The most pleasant way for a gank to be ruined is for the laners to simply kill the enemy before the ganker arrives. There is the issue of the ganker's wasted time, but there's no denying that a kill in lane puts your team ahead. If I go for a gank and the enemy lane decides to make a huge mistake before I get there, there are definitely worse things that can happen than my pals making the most of the mistake. An issue where a trade occurs (especially bad trades like trading a kill on your support for a kill on their ADC) is a bit worse, but basically the same thing. At least you get to turn around early in most cases.
If allies jump the gun on your gank, a cautious enemy will just respond to aggressive positioning by withdrawing. If the enemy was taking an aggressive stance, this is pretty frustrating. Usually this ruins the gank and neither you or your lane buddy get anything to show for it.
Sometimes an ally will engage before you get there and simply die. At least one failed assist gets on some streamer's top five failed plays every week. Consider the situation: I ping the lane to let my mates know I'm coming, and our top lane decides to engage at a disadvantage, dying before I arrive. When I do finally get there, the enemy is already backed up far enough that he has a safe run to his turret. At least I get the minion farm from the lane, but the enemy team gets ahead overall.
Sometimes "too early" means considerably after I've engaged. Often I have to make the hard decision of going for a gank when my ally is already badly injured and pushed to his turret. If I'm confident that I can win the fight on my own (perhaps the enemy is wounded too), I may decide to go for it. Unfortunately, my wounded ally frequently takes this to mean that he should "help."
Of course, the flip side of this is that my ally wants the kill or assist. There's nothing wrong with that, but if he really wanted to help, he should have not lost 80% of his life to harassment, poor engagements, or turret shots. Moving in at the last second to get an assist or even the kill is a heck of a lot better than throwing his life away and giving the enemy lane more gold advantage.
Asleep at the wheel
I try to ping a few seconds before my gank for a lot of reasons. The first, I've already explained: I want to minimize the chances of my ally jumping the gun. The second is that an On My Way ping is loud and obvious, and many lane allies will forget that I'm ganking if I announce it too early. The On My Way ping is very loud and says exactly what I'm intending to do and how I'm getting there. I'm yelling at my ally, "Wake up! I'm coming to gank!"
Sometimes this is simply not enough. Sometimes it is too hard for an ally to actually get into a position to do something useful. Yes, there is the issue of jumping the gun, but there are plenty of ways to play cautiously while still retaining the option to punish an enemy mistake. In fact, that's really how someone should be playing anyway.
Sometimes the enemy's only mistake is pushing the lane far ahead, and sometimes allies will be legitimately afraid of long-ranged harassment. When I get into the lane, I do expect them to go for broke. A smart enemy is running away, and I am a jungler and probably have some way of limiting his ability to fight. A foolish enemy that doesn't react is an enemy who will let me stun, slow, or otherwise cripple his ability to deal damage. That is the time to engage!
Frequently allies don't use their abilities, don't engage at all, make erroneous clicks (attacking minions instead of champions), or engage the wrong enemy. Sometimes when ganking a duo lane, the best target (frequently the ADC) reacts immediately, while his partner is unaware. Sometimes the support simply makes an aggressive play to try to save his partner. In these cases, you have to switch targets and go for the kill you can actually get.
Not every failed gank is your fault. Sometimes it's lack of information, but frequently someone just screwed up. It's extremely infuriating to gank a lane only to have my ally Garen die and the enemy Jayce get away.
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