Python 3.15 changed what open() does on Windows when no encoding is named, from the cp1252 code page to UTF-8. Running both versions on Windows 11, none of the 159,706 assigned non-ASCII Unicode characters written by Python 3.15 came back right when Python 3.14 read it. 82% came back wrong without any error, and the rest raised. In the other direction Python 3.15 raised on every character Python 3.14 could write. R moved to UTF-8 on Windows in 2022, so every R to Python handoff tested that broke under 3.14 works under 3.15, and pandas never changed at all.