Aargh! I hate it when people quote text as images because it makes it impossible to cut and paste.

> Jet black/Jet Blue ... catnap/dognap

My favorite examples are how prepositions can change the meanings of idioms. For example, to be "down for" something and "down with" something mean the same thing, but to be "down on" something means the opposite. (And going down to X means something very, very different from going down on X. That last example is also interesting from a geeky HN point of view because the preposition imposes a type constraint on the binding of X, which is why I had to use "X" instead of "something" :-)