mojibake
Corrupt characters or letters, especially resulting from being displayed or transferred through an inappropriate character set or encoding.
uncountable
Alas, you'll even see “text” files where some lines have one encoding but other lines have different encodings. You are guaranteed to see mojibake.
Where it comes from
Borrowed from Japanese 文字化け (mojibake), from 文字 (moji, “character”) + 化ける (bakeru, “to transform, take a different form”, generally in a negative way).
Adapted from the Wiktionary entry mojibake (CC BY-SA 4.0).