Moving platform
A surface that moves and is supposed to take whatever stands on it along for the ride, which makes it one of platform games' most underrated sources of bugs.
The problem is that the player is standing on something that moves without the player moving. Two approaches dominate: temporarily parent the character to the platform in the scene graph, or add the platform's displacement this frame to the character's position. Both require knowing when the character is genuinely standing on the platform, not just brushing its edge.
The classic mistake is update order. Move the character before the platform and it lags a frame behind, sinking into the surface or jittering on top of it. Platforms that descend faster than gravity leave the player floating, and ones that accelerate sideways force a decision about whether a jumping player inherits their speed. The platform itself is nearly always a kinematic body, because an elevator the player can shove out of the way is a poor elevator.