Python's handling of mutable default parameters presents a common pitfall for developers, particularly when functions behave unexpectedly by retaining changes between calls. This issue arises because Python passes parameters by reference, meaning that mutable objects can be unintentionally modified, causing side effects that persist across function calls. For example, when a mutable default parameter is used, such as a list, any modifications affect the original object, leading to unexpected behavior in subsequent function calls. The article illustrates these concepts through examples, highlighting the difference between mutable and immutable types, and the importance of understanding how variables reference objects in Python. To prevent unintended side effects, it is recommended to use immutable types or set default parameters to None and then initialize new objects within the function. This approach ensures consistency and avoids the pitfalls associated with mutable default parameters.