Dynamic Level Rearrangement

Dynamic Level Rearrangement (also known as DLR) is a major anomaly that occurs usually after sighting the White-eyed Chomp. The effects of Dynamic Level Rearrangement vary but it can only involve a level's textures, objects, text strings, and music. Nintendo's experimental AI is the main reason why this happens. It is able to modify existing textures but can't produce entirely new ones. The same applies to objects in the level, it can only modify and reposition existing objects. Rarer cases involve text strings getting changed, and also music swapping/rearrangement.

Documentation
There have been reports of DLR from players over the years, but there hasn't been any authentic evidence until recently. This player had discovered an instance of DLR when the star locked behind Chain-chomp's cave had been moved around to (what seems to be) the top of the mountain. A row of coins had replaced the star's spot. In the older written reports, claims were made that other levels and even the Castle itself was effected by Dynamic Level Rearrangement.

Theorizing
It's believed that Nintendo's experimental AI is attempting to create new level layouts that are geared toward the skill level of the current player. However, nothing entirely proves this theory correct as the code for the anomaly is obfuscated.