If you don't want to know the circumstances surrounding Sunday's entry in this slideshow, then don't read any further...
[spoiler]Sunday's quote:
Ep #797 (1897) - Quentin - 'Let's go, Magda, MUSTN'T keep our ever-hopeful cousin waiting.'
From "Robservations 1/5/03 - #796/797 - Quentin's Bacon is Saved; Another Curse"
someone knocks at the door. It's Magda--Barnabas wants you, and fast, she says--he has a plan. Beth asks what plan?--I'm coming, too. Magda says Barn only wants Quentin. Q claps Magda on the shoulder and says all right, Quentin he'll get--you must excuse me, Beth, I must beg off the next waltz. Beth starts to blather something about Barnabas' plan working. Quentin ignores her optimism and says, let's go, Magda, mustn't keep our ever-hopeful cousin waiting. And they go. Beth sits down in the chair Quentin vacated, looking as if she's going to cry. And the music plays.
[/spoiler]
The Robservations references the quote perfectly...
And contrary to the Robservations excerpt, Quentin does not say "must" where indicated before the quote comes up. What he actually says is, "My dear, you'll have to excuse me. I'm afraid I'll have to skip that next waltz."