What causes a 'cannot read property' error in Scratch?

A 'cannot read property' error usually occurs when you're trying to access a property of a variable or object that doesn't exist or is null. This often happens when you're working with lists and trying to access an index that's out of bounds (e.g., trying to access index 10 of a list with only 5 items). Double-check your list indices and ensure the list contains the expected data.