RNG

"LIMBO has zero RNG - until you get really good." -Sutiibu

RNG (Random Number Generator) refers to events that happen (almost) completely random in a game and cannot or only very hard predicted during a speedrun. Complete mathematical randomness is impossible in video games as of now, except you have true rng chip in your computer that gets its values from quantum physics level and the game was conceptualized for one of these. Usually, games get their RNG values from so called seeds that are mostly calculated from the system time at a certain point.

RNG in LIMBO
Contrary to popular belief, there actually exists RNG in LIMBO. While in early stages of running the game, the player will hardly notice it, after achieving a certain skill level, the few RNG that does exist can make or break a run. Especially towards the end, RNG events accumulate.

Piston RNG
The piston after the elevator can have a good or a bad cycle. Sometimes, you'll perfectly make the jump over it after pulling the boxes at the ceiling, but other times you'll need to wait an extra bit.

The Piston is NOT RNG. It is triggered somewhere after the machine guns/before the gravity boxes on the Normal Route. To get a good cycle, go through gravity boxes as fast as possible, maybe even using a Zetbox.

Hotel Light RNG
After you get the good jump from the ropeway, the hotel sign can flash in a good or a bad cycle. Optimally, you can walk up the plank and immediately jump after you reach the top. If in a bad cycle, you will amost need to wait a full cycle after you reach the top until you can jump.

It is believed (by glitch hunter Zet) that the light cycles are triggered somewhere in the game as well. That exact trigger hasn't been found as of yet though.

Rope RNG
Directly after the second hotel light, you will encounter a long rope that you need to hang on to. Usually it happens that you can walk down from the flashing "H", to hang on the ledge right next to you. Now, either the rope has a strong movement left and right or moves once to the right and remains almost still afterwards. If latter is the case, you cannot do a quick rope and have to rely on a safe strat by jumping to the "H" to your left again after it stopped glowing and from there back to the rope. If it still moves with a lot of momentum after that first move, you can simply let go of the ledge while the rope is moving to the right and the boy will grab the rope while falling down.