How to subscript/superscript single character in Text Edit?
The problem was that I had plain text turned on, instead of rich text . ( Format-Make Rich text ).
I don't know if plain text is the default option or not, but when you have plain text you can't use subscript/superscript single characters, it just applies the transformation to the whole line. Surprisingly, when you subscript something in rich text and then switch back to plain text, you get this popup:    Making a rich text document plain will lose all text styles (such as fonts and colors), images, attachments, and document properties.
I just don't get why these unsupported operations aren't greyed out when they don't work properly.