"Cracks" in the geometry mostly. These games have a few things in common, they have gravity and they have collision detection. These cracks are locations where the collision detection failed in some way. It could have been sharp edges, gaps or a number of other geometry anomalies. Could even been issues with time steps in the physics engine, where the updates desynced and the character was allowed to fall through perfectly good geometry. When the collision detection fails, there's nothing to push back from gravity, so you fall. Just like walking off a cliff.

It's not really "fixed". There are techniques to avoid this, but it's mostly just better tools and geometry creation tools for landscape so these anomalies aren't introduced.

"Cracks" in the geometry mostly. These games have a few things in common, they have gravity and they have collision detection. 

These anomalies are locations where the collision detection failed in some way. It could have been sharp edges, gaps or a number of other geometry anomalies. Could even been issues with time steps in the physics engine, where the updates desynced and the character was allowed to fall through perfectly good geometry.

This is common when the player does something that wasn't expected by the level designers. Jumping up onto some place they're not supposed to be, or somehow surviving a fall they weren't supposed to. Or, when the player uses features in the game like teleporting or flying to get to places the level designers didn't think players could get to. As recently as Skyrim (video: falling out of the world at 11:25 I suggest muting the audio...).

When the collision detection fails, there's nothing to push back from gravity, so you fall. Just like walking off a cliff.

It's not really "fixed". There are techniques to avoid this, but it's mostly just better tools and geometry creation tools for landscape so these anomalies aren't introduced.