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 #41 (1966) - Roger - 'Well, if you MUST know, I had a perfectly valid reason for being home-- I prevented your daughter from meeting Burke Devlin.'
From "#0041/0042: Robservations 06/15/01: Sam's Torment"
Roger is on the phone still, hand impatiently perched on his hip. He drops it back in the cradle, dials, then cuts off the call with a guilty expression on his face when Liz comes in. He claims he was trying to get the office on the phone, and she asks if that isn't where he should be. He faces her and says he had a valid reason for being home--he prevented Carolyn from meeting Burke. Why would Carolyn want to meet Burke? Liz asks. He claims that question is open to the wildest of speculation and she orders him not to be silly. She asks how he prevented the meeting, and he explains how Carolyn deliberately left her ring at the restaurant, where Burke found it. Carolyn had every intention of going back to get it from him. Liz pooh poohs this idea, but Roger, grinning, asks smoothly, "When you were a girl, didn't you ever accidentally on purpose leave something behind as an excuse for returning?" Liz imperiously asks her brother if he thinks Carolyn is really interested in this man, and Roger kindly responds that it's her daughter, her problem--he has his own with his son.
[/spoiler]
The Robservations references most of the quote well, though not perfectly...
And this is not only the second quote from this ep but it's also the second quote from this scene. The first (Roger - 'Well, if you MUST know, I spent it. But I had fun with my money, Liz. I enjoyed it. Can you say the same?') was back on February 13th.
And apparently "must" isn't the only favorite of Roger's, he's also fond of using it in the phrase, "Well, if you MUST know,"