These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fridge Brilliance: The April Fool mini game seemed rather random, with a luck-based fishing game to earn special fish-themed clothing... until you realise that in France, the holiday is called Poisson d'Avrile, and is based on sticking fish to other people's backs... GENIUS (and also strange).
Germans Love David Hasselhoff: It is originally a French game, but it is loved by the English speaking part of the Internet so much, they already had to make a second English server. There is also a significant Brazilian fanbase.
Good Bad Bugs: Some call the walljumping technique a glitch. Others call it a feature. There's even a map that makes direct use of the physics glitches to create a two-armed monster made of connected anvils for the mice to overcome, known as the Anvil God.
You used to be able to spawn a portal (in any level) on screen AND off screen, pushing a mouse into said on screen portal will immediately turn said mouse into bubbles. Needless to say, dick Shamans loved this. Then they removed the portals (but re-used them later on), probably because of this and the fact Shamans only used portals to easily beat a level.
In one version of the game, objects that are supposed to be pinned to each other at the start of the level, fell apart instead. While this made one level unplayable, it also made a lot of them much easier.
Several techniques were quickly found to survive the red wood. Whether sliding onto it and simply walking across, or double jumping as soon as you touch it to eliminate the vertical momentum...
Goddamn Boss: The Halloween Event's giant (skeletal) cat boss.
Memetic Mutation: The dreaded anvil monster/god created by anchoring multiple anvils and boards together and setting some of them to rotate. An official level later turned it into an Ascended Meme by using it as a pseudo-Boss Battle. There's even Rule 34 of it already.