UnicodeDecodeError: codec can't decode byte

Python 3.x is not uniformly backward compatible — significant changes between minor versions cause Unicode Decode Error when code written for Python 3.8 runs on 3.11, or vice versa. Key changes by version: Python 3.8 introduced the walrus operator (:=) and positional-only parameters — code using these features fails on 3.7. Python 3.9 added built-in generic type hints (list[int] instead of List[int] from typing) — breaks on 3.8. Python 3.10 introduced structural pattern matching (match/case) and better error messages for tracebacks. Python 3.11 brought significant performance improvements and changed exception group handling. Python 3.12 deprecated several datetime behaviors and renamed internal APIs that some packages used directly. Always specify the minimum Python version in pyproject.toml or setup.cfg with python_requires = ">=3.10" to prevent installation on incompatible versions. Test with tox or nox across multiple Python versions to catch version-specific Unicode Decode Error before deployment.