> It would be as though people started using "the die is cast" to mean dying, simply because the word "die" is in there, without any knowledge of Caesar. Is there any idiom -- not a single word, but a full phrase -- whose meaning has changed over the years, simply by virtue of its being misunderstood by the linguistically inept or the historically ignorant?

Ironically enough, Caesar’s quote of Menander likely meant “this crossing of the Rubicon is a roll of the dice”, or “let the dice be thrown”, not that his crossing is setting in motion the irrevocable. So there’s that.