Also known as: 文字化け
Mojibake (literally "character transformation") is a term for the garbled or Gibberish Text that results from text being displayed in the wrong encoding. This is a common problem in Japanese, where text may be written in one format, for example Shift-JIS, but displayed in another format, such as Unicode.
Sometimes, if the font is lacking appropriate glyphs, a substitute character might be shown (such as blank box, or particularly egregious, a ? in Microsoft products, making it extra-hard to notice that something is missing in small snippets).