Quicksand is a deadly surface that instantly kills the mighty plumber on contact in Super Mario 64. The only way for Mario to survive this is if he performs a slide kick, allowing him to bounce on the surface once before again being killed unless he reaches a safe surface.
In Super Mario 64, it only appears in Shifting Sand Land and is clearly marked(as a darker patch of sand or moving sand) and is relevant to the theme of the level(a desert). However, in many hacks, particularly in garbage ones, it is constantly spammed to death, creating giant areas of death that only Simpleflips, BUP, and Mario can pass with extreme skills. It is common in these ROM hacks that the quicksand is unmarked(often as a surface that is the exact same material as a material that isn't quicksand, like a roof), leading to mysterious and often times unfair death, causing uproar for everyone who dies from it.