A ligature is a combination of letters that meld into each other, such as ff or fl or ri in some fonts (LaTeX is one of them). In math mode, the Latex puts quite a bit of space on each side of [unparseable or potentially dangerous latex formula] (commonly used for functional notation) and as such, LaTeX ff looks like [unparseable or potentially dangerous latex formula].
/puts away shotgun. The joke is dead.
~MP