Fairly obvious, in retrospect...

Why you shouldn't mix Gilbert & Sullivan and Princess Bride:
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I'm called Princess Buttercup, soon to be Florin's Queen,
Though I could never tell why -
Prince Humperdinck hates me - ah, poor Princess Buttercup,
Soon to be murdered am I!

I was dragged into danger by three deadly strangers,
And some shrieking eels tried to chomp -
But then - inconceivably! - Man in Black rescued me,
And carried me into the Swamp.

We faced perils dire, from Snow Sand to fire,
And ROUS's defied;
We lived - were elated, but Humperdinck waited,
In the Count's machine Westley has died.

But he's my dear Westley, and I am his Buttercup,
And our love will not fade away -
I betrayed him and shunned him; though it may have stunned him,
He'll save me from my wedding day!