The question of failed watering seems to hold people's attention for the last couple of weeks, so I went through my recordings analyzing my unsuccessful pours.
TLDR: It's all about speed - when you water plants in pairs, if you click too quickly after turning away from the previous plant you're guaranteed to fail.
Now let get into details.
#1 - The fastest fail:

As you can see I turn and click so quickly, that even though the target reticle is on the correct plant, the highlight and watering animation are actually behind the guy's back, still on the previous plant. Going frame by frame through several recordings, it seems
there is only one frame between those plants, where I'm not watering. On that first frame, after I finish watering, the guy is already facing the opposite direction, but the target reticle is still behind his back and only moves to his front on the next frame, so when I manage to click during that frame, the game correctly pours water on what I'm targeting but it looks wrong because the frame where the process starts is also the frame where the target reticle goes to its new position.
It's a frame perfect trick that you don't want to perform
#2 - The second fastest fail:

Basically same principle, only the "travel" between waterings takes
two frames. Now that one is less clear, since the target reticle and the animation are in the right place, but the pour is still unsuccessful. This behaviour and similar examples in hoeing challenge make me think that the actual game logic is lagging about a frame behind what we see.
Which brings us neatly to...
#3 - The fastest success:

There it is - 3 frames between watering. When I get into proper rhythm, I seem to hit 5-6 frame interval most of the time, with occasional 7 or 4.
And now some extra bits.
#4 - Sometimes you can mess up a two frame turn even worse.

Somehow I manage to change direction again before clicking, so I'm not only pouring water too soon, but I'm also doing it behind my back
#5 - WTF is going on there?

The plant is highlighted for a couple of frames before I click, but the target reticle only moves to the plant tile on the same frame as the watering starts, and the water animation shows up on the empty tile between plants
and yet the pour is successful 
This example kind of breaks my theory from #2 about game logic consistently lagging a frame behind visuals, but it feels to good to discard and this thing here seemingly happened to me only once. Maybe the game's targeting is not always lagging, but just lives somewhere around what we see? I have no idea anymore
In conclusion, at least one thing is clear: watering earlier than 3 frames after finishing the previous one is to be avoided.
Initially I thought that only case #1 is caused by turning speed and that case #2 might be due too some desync in the overall "pour-turn-pour-turn" cycle. Like for example the plant only becomes watered at the last frame of 34-35 frame process, so since I almost always start watering a plant before it transforms and therefore can accept new water, I thought that perhaps it needs to be watered for some minimal amount of time for it to work. But no - some of my pours failed after 25 frames of watering transformed dry plant for 25 frames and some succeeded after only 17 frames.
PS You might wonder why I spent hours going through recordings, counting frames and trying to come up with explanations for how a small minigame works.
The answer is simple:


